Yes, they went back to the roots. Studied all the great blues men. Back to the earliest recordings. They probably even listened to the Alan Lomax stuff, the Georgia Sea Island Singers doing the old slave songs etc.
If you think about what the early black music represented( black people putting to music the frustrations of being lorded over by their masters), it was brilliant strategy to use the dissonant cords and progressions that made up the blues distinctive sound and adopt it for the leftist agenda. It turns out to be ironic that the leftist agenda now is becoming so successful in lording over so many people.