Who said blah blah blah on the peanuts comic strip?

This means that an adult is speaking .

Charles Schultz never showed an adult speaking in the comic strips -- this is only from the TV Cartoons, where you heard a horn-like "Wa-Wa" noise whenever an adult was speaking.

"Blah blah blah" in slang is a way of saying that you've heard all that's being said so many times that it's boring to you.