The origin of 'blah' in the sense of meaningless talk
Eric Partridge, A Dictionary of Slang and Unconventional English, second edition (1938) has no doubt that the actual word blah is of U.S. origin, although he then indulges in a series of etymological conjectures that amount to an embarrassment of riches:
blah, n. and adj. Nonsense; silly or empty (talk): deliberately wordy, insincere, window-dressing (matter): 1927, esp. among publisher and journalists. From U.S., where it existed in 1925. ... Perhaps ex Fr. blague, but more prob. ex Ger. s. Blech, nonsense, there being millions of Germans in 'the States'. More prob. still is derivation ex Scottish and Irish blaflum, nonsense, idle talk; Ulster has the variant blah flah.
In the fifth edition of his book (1961), Partridge appends a final option to the foregoing etymological possibilities:
Or, of course, merely echoic.
Right. Partridge also has this interesting entry for go blah:
go blah. ... Prob ex:—2. To have one's mind go blank: from ca. 1907: Parliamentary >, by 1930, gen. A.E.W. Mason, The Dean's Elbow, 1930, in reference to the year 1908 and to a prospective speaker in Parliament, 'If only his mind didn't go blank. Minds often did, even the best minds. Darkness descends on them, inextricable ... These seizures ... always chose ruinous moments. There was a slang phrase which described them—horribly graphic, too, like most slang phrases. To go blah. Well, there it was! He, Mark Thewless, would go blah that afternoon.' Perhaps blah represents a perversion of blank.
Regardless of how legitimate this remembered slang term retrospectively attributed to the year 1908 may be, the spelling blah belongs to 1930.
J.E. Lighter, Random House Historical Dictionary of American Slang (1993) has entries for various meanings of blah:
blah n. & interj. 1. empty talk, drivel. Also blah-blah{-blah}. [Earliest two citations:] 1918 in [Howard] O'Brien Wine, Women, [and] War[: A Diary of Disillusionment] 136: Pulled old "blah" about "service," "doing one's bit," etc. 1920 In C[arl] Sandburg Letters 186: But he's...always springing important copy book...TRUTHS about making the world better, the hire life, blaa blaa. ... 2. pl. a blasé condition resulting from boredom, minor illness, etc.—constr[ued] with the {Popularized and perhaps coined in an Alka-Seltzer advertising campaign, ca1967.} [Citations from 1968 and later omitted.]
blah adj. disappointing; unsatisfactory; dull; insipid; lifeless. [Earliest two citations:] 1922 Variety (Sept. 1) 5: I can drive like an old timer but my direction is very blah. 1922 in DN V 146: Started perfectly blaah, though.
blah v. to gossip, talk idly. 1928 Dahlberg Bottom Dogs 250: He could blah with a newsie.
O'Brien is Howard O'Brien, Wine, Women, and War: A Diary of Disillusionment (1918), which uses blah on three occasions besides the one that Lighter cites (which ids from an entry dated July 3, 1918):
Thur., [February] 14th [1918].—Swell mess of "interrogation" on crank-shafts. Tractor work all aft[ernoon] in mud. Sanctity of toil, etc.—blah!
...
Sun., [August] 25th [1918]. ... This all sounds like a "Y" Sec'y, 4 days over! Blah, perhaps. Not expressing what I feel. Words can't damn it!
...
Mon., [December] 9th [1918]. ... To H.Q., G2, and sprang great idea about substitute. Fell flat. "Done too well" and more such rot. Vague blah about getting "Most important post in A.E.F.," and promotion. Extreme value due to intimate and cordial relation's established with French and British— and more tosh.
O'Brien was an American novelist and journalist who lived in Chicago his entire life aside from his college years at Yale University and his service in France during World War I. He speaks familiarly of Carl Sandburg at one point in the diary, as though he knew him personally.
Sandburg no only used blaa blaa in a private letter but in a poem. Here is an excerpt from "Aprons of Silence," in Sandburg's collection Smoke and Steel (1920):
Only the keeper and the kept in the hoosegow/ Knew it—on the streets, in the post office,/ On the cars, into the railroad station/ Where the caller was calling, "All a-board,/ All a-board for .. Blaa-blaa .. Blaa-blaa,/ Blaa-blaa .. and all points northwest .. all a-board."
In fact, other Sandburg contemporaries associated him with the term. Ben Hecht, Erik Dorn (1920) refers to Sandburg as "old Carl three times, including this relevant instance:
"... The woods are full of smart alecks like you and they make me kind of tired, because I can never figure out what they're talking about. And I'll be damned if they know themselves. They think in big hunks and keep a lot of words floating in the air. ... What old Carl calls 'Blaa ... blaa. ...'"
The two friends sat regarding each other critically. Dorn nodded after a pause.
"You're right," he smiled. "I'm part of the blaa-blaa. I heard them blaa-blaa with guns in Munich one night. And up in the Baltic. You're right. Anything one says about absurdity becomes absurd itself. ..."
Another impressive early instance of blah in a sense that combines onomatopoeia and the specific meaning "drivel" appears in Edward Teufer "Postlude, When the Dead Awoke," dated December 28, 1921, in The Nation (January 18, 1922):
On that day/ When first I read The Nation's prize poem/ And I said:/ Blah! ... Blah! ... Blah-blah!/ And my friend, Jim the iceman, coyly cooed:/ Phooie! ... Phooie! ... Phooie-Phooooieeee!/ [...]/ And I shall belabor the greasy dish-pan with grandma's old soup ladle!/ Wang! ... Twit! ... Wang! ... Twit!/ Belabor! ... Belabor! ... Splop! ... Splop!/ Blah! ... Blah!/ Phooooieeee!
Other early senses of 'blah'
The first matches for blah in the sense of "dull or lifeless" are almost as old as the first instances of blah in the sense of "blather." From "Flapper Filiology—The New Language," originally in the Philadelphia [Pennsylvania] Evening Bulletin, March 8, 1922, reprinted in Dialect Notes, volume 5 (1922):
Flapper—"Hot dog! It was the cat's pajamas. Started perfectly blaah, though. Joe brought a strike-breaker, some tomato he turned sub-chaser for, 'cause his regular jane had given him the air. Jack had a flat-wheeler along who was a cellar-smacker. He got jammed."
[Explanation:] Blaah!—anything that's no good or is "out."
None of the entries that Lighter offers for blah seem exactly on point for the instance in Jack Bechdolt, "See America First," in Argosy-Allstory Weekly (August 21, 1920):
Bruce Ray had become a lounge lizard, a cabaret hound, a wrist-watch whiffet, a blaah, blaah! The champion log spinner of Whatcom and Snohomish Counties, and the best boomtender in the Pacific Northwest had been spoiled to make positively the worst imitation of a summer-resort nothing-at-all that even the worst summer resort on Puget Sound ever had seen.
The meaning here seems to be "a boring layabout consumed with trivial amusements and momentary distractions."
Yet another early instance of blah uses the term as, in effect, a synonym for "name redacted." From "Radio in the 'Dry' Navy: Boats of the Rum Chasing Fleet Are Equipped with Modern Spark and Telephone Transmitters—Orders and Reports Go by Wireless," in The Wireless Age (November 1922):
Chaser: "Who are you?"
Smuggler: "Schooner Blaah, out of Nassau for Halifax."
C. "What is your cargo?"
S. "Wines and liquors."
C. "Where do you think you are?"
S. "Four miles off the American coast, on the high seas."
C. "You've got another guess coming. You're half a mile inside American territorial waters. We'll send a custom official aboard."
And in his little radio cabin, the operator sends back to headquarters the report: "Boarded schooner Blaah, Nassau for Halifax, with cargo of liquor, anchored in territorial waters. Proceeding to New York with her." Another prize has been captured.
All of the preceding examples have been of U.S. origin, but there are two much earlier instances of a similar word/sound from England. From James Halliwell, A Dictionary of Archaic and Provincial Words, Obsolete Phrases, Proverbs, and Ancient Customs (1847), volume 1, we have this notice of the archaic term blaa:
BLAA. Blue. Yorksh[ire]. Applied more particularly to the appearance of the flesh after a heavy blow. [Example:] And bett hym tllle his rybbis braste,/ And made his flesche fulle blaa. Sir Isumbras, 311.
And from William Dickinson, A Glossary of the Words and Phrases of Cumberland (1859), we have this amusing entry for a regional pronunciation:
Blaa, S.W. blow. A postman overtook a butcher, leading a fat calf by a cord tied round his own waist, and vainly endeavouring to get the animal to cross a footbridge on the path. The butcher requested the postman to blow his horn when the calf was got into a favourable position, and on giving the word of command to "Blaa, Jimmy, blaa!" a loud and sudden blast was given, and over went the calf into a deep pool, dragging the butcher along with it.On recovering his feet, he turned to the astonished postman, vociferating, "Thaww fooal, thaww! that's far oor big a blaa for a fat cofe!"
These examples underscore the simplicity of the sound blaa or blah, and the similarity it bears to other English words, both related in sense (blab, blasé, blather, blither) and utterly different (baa, bah, blow, blue, blew).
A theory of inarticulate origin
Writing in 1936, H.L. Mencken, The American Language, fourth Edition, places blah in a category of words that in his view are not derived from any meaningful prior words in English or any other language:
The latter [the comic strip artist, brought into journalism by Joseph Pulitzer and William Randolph Hearst] has been a very diligent maker of terse and dramatic words. In his grim comments upon the horrible calamities which befall his characters, he not only employs many ancients of English speech, e.g., slam, bang, quack, meeou, smash and bump, but also invents novelties of his own, e.g., zowie, bam, socko, yurp, plop, wow, wam, glug, oof, ulk, whap, bing, flooie and grrr. Similar onomatopeic forms of an older date are listed in the Supplement to the Oxford Dictionary as Americanisms, e.g., blah, wow, bust and flipflop. All these, and a great many like words, are familiar to every American schoolboy.
Mencken points readers to an excellent article by E.C. Hills, "Exclamations in American English," in Dialect Notes, volume 5, part 7 (1924). In Hills's categorization, blah falls under the heading of "articulate interjections," along with such companions as ah, ahem, ahoy, bah, boo, bosh, faugh, fie, hoopty-doo, hurrah, la-de-da, num-num-nummy-num, oh, ouch, pish, pooh, sis-boom-bah, whoah, whoopsie, and yoo-hoo. This category is one of three in Hills's system, the other two being "inarticulate exclamations" (such as laughter, weeping, shrieks, groans, grunts, and whistling sounds) and "onomatopoeic words" (such as bam, boom, bow-wow, honk-honk, meow, and moo). Hills describes the "articulate interjections" category as follows:
It is probable that some of these were originally inarticulate grunts or clucks. If such is the case, attempts were made to express them after a fashion in writing, and the resultant forms became standardized. Thus "ahem" is probably an attempt to express in writing an inarticulate nasal grunt, but nowadays a reader pronounces it as it is spelled.
As for blah in particular, Hills offers this brief entry in his "General List of Exclamations:
blah: (triumph or disg[ust]); —-— (onom[atopoeia]: com[ic or humorous])
We see an unvarnished example of onomatopoeic use of blah in the first Google Books match for the term, in Harold Susman, "Mrs. Billie's Baby," in The Smart Set (June 1911):
"Just listen to this, said Mrs. Billie. "Augustus, how does the bow-wow go?"
"Blah! Blah!" said Augustus.
"See!" said Mrs. Billie. "He knows!"
"Wonderful!" said Mrs. Van Martyr.
"Augustus," said Mrs. Billie, "how does the pussy cat go?"
"Blah! Blah!" said Augustus.
Augustus subsequently identifies "Blah! Blah!" as the sound a poll parrot, a choo-choo car, and person saying goodbye make, too. Ultimately, blah is an expression of infant babble without underlying sense.
I think that Hills's (and Mencken's) analysis has common sense on its side. An expression of that originates as an onomatopoeic transcription of an inarticulate exclamation or sound seems peculiarly susceptible to having multiple meanings attributed to it, which appears to be the case with blah. If we examine blah in the context of many similarly regularized forms of interjections, we may see that it is far from unusual: English has many such words. The fact that usage has refined them to the point at which they have a standard articulated form and specific meanings obscures their probable origin as sounds without meaningful content—the message of the baby Augustus in 1911.
Conclusion
It is tempting to imagine that we owe the word blah in contemporary English to the literary efforts of Carl Sandburg or, alternatively, to the corruption or transformation of any of a number of English or foreign words. Nevertheless, I think that the more likely source of blah is imitative of an originally meaningless (or at least untranslatable) sound—much as the duplicate syllable βαρ-βαρ emerged in ancient Greek to represent the gibberish sounds made by non-Greeks.
Best Answer
You already have the OED's earliest cite, which points out that it seems to have originated in Liverpudlian, not London, English. There's no present understanding about the term, though you could try looking for resources on the history of Scouse that might earn you a place in future etymology compilations.
The only thing I'll note that others haven't is that there is no Latin term contango except there is: contingo is formed from con- + tango, with a vowel shift occurring. The term could have originated in dog Latin (it's irregularly attested on Google Books in the 17th and 18th century) and simply proven too profitable a notion to not have a ready word at hand.