There is a semantic difference between look/appear and seem:
One should use look or appear when describing an observable condition - e.g.: Rhonda looks sad - the example implies that there is some observable state or behavior that supports the statement (Rhonda may have tears rolling down her cheeks, for example).
One should use seem to describe a perceived condition - e.g.: Rhonda seems sad - the example implies that the person making the statement has a perception regarding the condition of the subject.
From New Oxford American:
dark horse |ˈdɑrk ˈˈhɔrs|
noun
1 a person about whom little is known, esp. someone whose abilities and potential for success are concealed : [as adj.] a dark-horse candidate.
2 a competitor or candidate who has little chance of winning, or who wins against expectations : a preseason dark horse as the nation's top collegiate football team.
sleeper |ˈslipər|
noun
3 a movie, book, play, etc., that achieves sudden unexpected success after initially attracting little attention, typically one that proves popular without much promotion or expenditure.
• an antique whose true value goes unrecognized for some time.
From Etymonline:
dark horse
in politics, 1842, an image from horse racing, in which dark is used in its figurative sense of "unknown."
Moonraker is called a "dark horse"; that is neither his sire nor dam is known. ["Pierce Egan's Book of Sports," London, 1832]
sleeper
[...] Sense of "something whose importance proves to be greater than expected" first attested 1892, originally in Amer.Eng,. sports jargon, probably from earlier gambling slang (1856) sense of "unexpected winning card."
Here's some generalizations gleaned from the above:
- dark horses are usually people; sleepers are usually things.
- dark horses are intentionally kept unknown; sleepers are simply not promoted.
- a dark horse might achieve success; a sleeper already has—surprisingly.
- dark horses' limelight often decrescendoes; sleepers' crescendoes.
As to your questions, I'd say they're not quite interchangeable. Knowing their different connotations will likely prove one to be the better word choice.
A dark horse might also be called an enigma, or simply an unknown; while surprise hit, cult classic, and box-office success are other phrases often applied to sleepers.
Best Answer
Look at suggests that you already know what your options are and will review them, whereas look into suggests that you first need to investigate what those options might be.