Green’s Dictionary of Slang

(US campus) an excl. of frustration, boredom or resignation.

1922		N.Y. Tribune 26 Oct. 8/5: When he began to scold she began real loud to go, ‘Blah, blah, blah, blah’.
1947–53		W. Guthrie Seeds of Man (1995) 236: Blahh! She stomped her feet around and around my blanket [...] Blahhh on you!
1976		Jenkins & Shrake Limo 236: ‘I hope seeing Jeff was worth getting caught,’ I said. Cindy wrinkled her nose and said, ‘Yuk. Jeff. Blaaah’ .
2001		Eble Campus Sl. Fall 1: blah – expletive of boredom or resignation to an undesirable task.