Stories We Like: A Guide to the Comma
"idle, meaningless talk," 1918, probably echoic; the adjective meaning "bland, dull" is from 1919, perhaps influenced by French blasé "bored, indifferent." The blahs "depression" is attested by 1966.
An expression of disagreement, contempt, etc; bullshit (1920s+)noun
[echoic; but first sense said to be fr French blase´, ''indifferent, bored'']