It is not funny. Another view: It depends on the context in which you use the word, "blah". Blah means worthless or silly talk. Lots of jokes can be made using the word, but it can also be used in...
for the sake of ke$ha's career...:)
This is a hard question and depends on your opinion, but most people would say no. Onomatopoeia is a sound, and "blah blah blah" is something we say.