Good question! It's obviously a blues thing, and I'd guess it started with instrumentalists imitating blues singers. Blues is a vocal music, and employs a scale which naturally bends the fixed notes of western music (it's a combination of African and European folk practices, beginning probably in the late 19th century).
Obviously when instruments play the blues, they try to imitate the voice. Pianists obviously can't do it, but horns (trumpets, saxes, etc) can, and guitars certainly can. In the earliest jazz and blues recordings you hear it being done.
It's not breaking any rules - it's following the rules of blues - and I doubt very much that someone discovered it by mistake and thought "hey that's cool". They were trying to sound like blues singers.
__________________
"There is a crack in everything. That's how the light gets in." - Leonard Cohen.
|