Sense “Idle, meaningless talk” (1940), probably imitative or echoic in origin. Perhaps, but cf Greek "barbarbar” ‘unintelligible sounds’ (Grillo 1989:174).
Adjective sense “bland, dull” (1919), perhaps influenced by French blasé.
The blahs (“boredom, mild depression”) first attested 1969; extension of adjective sense and influenced by term blues.