Green’s Dictionary of Slang

1. (orig. US) insincere, verbose, pompous.

1925		R. Lardner ‘Zone of Quiet’ in Coll. Short Stories (1941) 73: He brought along his B.F. and another girl. She was just blah, but the B.F. wasn’t so bad.
1940		M. Marples Public School Slang 15: blah blah (Rugby, 1926+), elsewhere commonly blah-blah, from blasé applied to affected speech and behaviour.

2. askew, wrong.

1920		Ade Hand-made Fables 129: The Fig Cake was a Triumph and the Jelly Cake held its Shape but the Hickory-Nut Cake went Blah.
1963		M. Braly Shake Him Till He Rattles (1964) 122: Oh, the bottles are so blah [...] But the texture of the soup cans is really quite interesting.

3. blind drunk.

1984

Partridge DSUE (8th edn) 91/1: since 1930.

4. (also blasé) uninterested, non-committal.

1960		‘Andrew Shaw’ Adulterers 105: ‘You happy?’ ‘No.’ ‘You sad?’ ‘No.’ ‘Just blah, huh? You just feel sort of blah, Sutton? Is that it?’ ‘Yeah,’ he said. ‘That’s about it’.
1968		J. Hersey Algiers Motel Incident 305: All of which [...] came to a blah ending.
1977		C. McFadden Serial 96: Beige is blah, Harv. It’s a nothing colour.
1997		L. Pettiway Workin’ It 209: She’d send us to school and ask us what we want to take up, blasé, blasé, and then she make us take up what she wants us to take up.

5. banal, cliched, undistinguished.

1922		Ogden Standard Examiner 12 Apr. 6/5: Mother— ‘Well, dear, did you have a good time last night?’ Daughter— ‘Oh, Mom, it was perfectly blaah — nobody there but a lot of cake-eaters and grease balls’.
1963		A.S. Fleischman Venetian Blonde (2006) 155: I wondered about the man with the blah face. Maggie’s description didn’t help much. The counterman had a blah face.
1999		J. Ridley Everybody Smokes in Hell 15: Did mansions have anything as blah as a front door?
2011		Times Mag. 30 Apr. 57/3: ‘You don’t think it’s blah?’ ‘No’.