How do you spell "blah blah blah"?

Any way you want. And that's official. :)

Remember that dictionaries neither define words correctly nor list correct spellings -- dictionaries dispassionately describe how people use and spell words. To see whether this is true, look up the word "literally":

Dictionary.com: literally:

http://dictionary.reference.com/browse/literally

1. in the literal or strict sense: What does the word mean literally?

4. in effect; in substance; very nearly; virtually.

How can both definitions be authoritative? -- they contradict each other.

Another dictionary, Merriam Webster, defines literally:

http://www.merriam-webster.com/dictionary/literally

1 : in a literal sense or manner : actually <took the remark literally> <was literally insane>

2 : in effect : virtually <will literally turn the world upside down to combat cruelty or injustice — Norman Cousins>

Same problem -- two definitions of the same word, but that flatly contradict each other.

Conclusion? Dictionaries do not prescribe, they describe. Their sole purpose is to report how people use words.

So spell "blah blah blah" any way you like. The only issue is whether your reader understands you.