Is "blah blah blah" the most common spelling?
Solution 1:
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.
Solution 2:
Just for giggles behold the Google Ngram:
"blah blah" is clearly more common.
Due to feedback, here's another silly metric:
Google search results:
bla bla: about 54,100,000
bla bla bla: about 36,300,000
blah blah: about 68,800,000
blah blah blah: about 54,400,000
Not exactly conclusive, but blah blah still wins... see hippietrails comment, he is better at googling than me.