The phrase "blah blah blah" and the single word 'blah' are both very informal. In fact, even though the OED is pretty descriptive, I'm surprised it has an entry for 'blah' (it is not something I expect in print, and that's all that OED relies on).
As to what constitutes a standard, for English, there is no government supported official body, like the French Academy, which dictates usage. It is a little more decentralized in English writing culture, relying on style guide writers (from book or newspaper publishing houses or self declared but recognized experts), and the primary and secondary school systems.
The phrase is informal enough so as not warrant an official, correct spelling by any authority. Because of its informality, one would not expect a magazine or newspaper editor to regulate its spelling because they would just try not to have it appear at all.
This might seem disingenuous because after all it is in the OED and there are many instances written on the net. Some people do write it. But the authorities on what should be written would probably say that it should not be written at all.
Then it falls to practice. And only practice defines (circularly) what is the most common. And that seems to be 'blah blah blah'.
Your friend 'corrected' you by telling you what he's seen more often. 'correct' and 'common' are not the same thing, but when there's no correctness authority it is all we have to go on.
As to whether two or three repetitions, I've never heard or used less than three in speech; if you're going to spout nonsense, might as well go all the way.
I've done nothing but sit on my rear all day trying to find you an appropriate answer. I've only come across one article online that seems to collectively dictate anything and everything that I've being reading. It seems that Canada defines the majority of its culture upon its language (and spelling).
While I have to agree with Robin Michael that you'll never really find yourself with a simple answer, I do hope this article may shed some more light.
According to him, the Canadian language, the culture, is being slowly diminished by your friendly neighbors to the south of you. (Hello!)
The Canadian culture was to be unique and different but outside factors have created mass confusion on how to spell.
(There were many other websites that mentioned bits and pieces of what this article says. I chose to link you this certain article, alone, because it was the only article to contain a bit more information closer to what you're asking.)
I posted this too soon it seems. I found this article that seems to have a somewhat similar take to the other article, but with a semi-different spin.
Taking your point to heart, I decided to refine my search, looking strictly for only books. Finding this book in particular, the Google Preview looked promising. There is a lot to read, however. I don't think you'll mind that though; you seem to really want this answer!
Soft edit: I normally dislike Wikipedia, and I don't know if you saw this or not, but I think it gives a general clue as to how Canadian English became mixed. Link
Best Answer
The answer to why it's such a commonly misspelled word is likely due to the fact that it's been being misspelled ever since its inception as a word.
etymonline shows it originating in the mid-15th century, but there are books that have the misspelled version dating all the way from the 16th century, such as in this French book from 1581:
Legende de Domp Claude de Guyse, abbé de Cluny: contenant ses faits et ...
or from this Scottish book from 1585-1592, which has three instances of the misspelling, so you know it's not just a typo:
Scotland. Privy Council - 1585
Google Books shows the misspelling thriving from the 18th century-on:
An interesting thing that can be picked up from the sources is that in the mid-18th century the misspelling made its appearance in many French texts, and an Ngram analysis of French documents confirms this:
Google Ngrams French
Now is this a case of the British misspelling marking its influence on French soil, or was the misspelling French in origin all along? I doubt the latter, but maybe someone else can shed some light. French was the lingua franca during the 17th century so its possible.
There are various English words that derive from Latin words beginning with "exh-":
exhonorate is another common misspelling of exonerate, but that has a Latin root beginning with "exh", unlike "exorbitant" and "exonerate".
https://en.wiktionary.org/wiki/exhonorate
My guess after all this evidence is that the medieval Brits thought that "exorbitant's" Latin roots had an 'h' in them, which made it a common misspelling at the time, and that misspelling survived through the ages. Truly a misspelling for the ages.