I remember our guitarist many years ago practising scales etc. He then played the jazz scale and some weird chords. "Once I have learned jazz scales and chords he said I can play anything."
Regardless of what genre you adopt a great debt is owed to jazz.
Graham could be right. Black slaves developed gospel from a mixture of their own beats and traditional hymns. I believe their own music was banned by the oppressive hypocritical Christian slave drivers as being unholy and possibly rebelious. Blues was born from early gospel and then Jazz came from blues.