This is a great subject for discussion, because it is an important question, and there aren't very many people who have examined it--

I have a couple thoughts, first, that it is important to recognize that a lot of what we recognize as gospel and blues/rhythm and blues is commercial music, which doesn't mean that it is bad, or even that it is somehow less valid than non-commercial stuff, but it does mean that it was created to satisfy a demand--

Second, that the commercial music replaced a variety of local traditional music traditions, and was also adopted in a lot of places where there had been no musical traditions--mainstream black congregations, before the introduction of the Thomas Dorsey style gospel music, tended to use the same music and the same singing styles as their parent denominations, and, it is worth noting that even now, "Gospel" is only a part of the worship of certain denominations--

Third--there seems to be a performance/creative mechanism/technique that underlies the seeminlgy unconnected religious music, secular dance/entertainent music, and work music traditions, and there is a lot of back and forth between the three areas--

Fourth, even though the outward forms seem to have radically changed over time, the underlying techique(and it's component elements) seems to be unchanged--