Tahir: See my guess above. What I'm pretty sure didn't happen is that it evolved in some direct way from work songs of the slavery days, as has often been suggested. Yes the call and response type of thing, which you find in blues, had that sort of origin, but that's obvious and trivial, since that is a characteristic of African music in general. I read an article recently about WC Handley and the first time he heard the blues. He was apparently on a station waiting for a train and a poor black man with a guitar near him (and dressed in rags) started playing this very strange music by sliding a knife on the strings. He recounted that it was like nothing he had ever heard before. As a black man who was already trained in classical music he realised that this was something different from both western harmony AND African folk music. For those of us who grew up in the post-blues era of rock music, other harmonies often sound relatively bland now.
^^^^^^^ CB: Angela Davis points out in a lecture on the blues tradition ( I have a copy on tape), that blues represents a change from the music in slavery in that blues discuss the new experience of Black women and men having relatively liberated romantic relationships under freedom as opposed to under slavery.
In the tape, Davis claims that music is more integral to everyday life and work in African culture than in European culture. This is part of African culture carrying over to African American culture in that music is a bit more integral to everyday life for Blacks.
As far as "folk" music, in the sense of music of agricultural workers, with rocknroll and Motown , et al. the urban worker factor enters. No doubt the sounds of factories and their rhythms impact Motown's sounds and other urban blues sounds. Berry Gordy was an autoworker. Proletarian music.