I have been meaning to read Michael Harris' book, "The Rise of Gospel Blues: The Music of Thomas Andrew Dorsey in the Urban Church" for a couple years now, and therefore have no way of knowing how good it is, but it probably is very good background for your effort--

Even still, his music is very commercial, he, like Handy before him, was a formally trained arranger and composer, and had worked for Paramount Records--he set the first independent, black owned music publishing company, and founded the National Convention of Gospel Choirs, to help market his music--

I mention this because this music, as good as it is, squeezed out a lot of the traditional gospel singing and performance styles (Lomax gives a wonderful description of an old time gospel service in "The Land Where the Blues Began"--some of this stuff survives, though very marginally--and some of it has been recorded, both by folklorists, and for commercial purposes, and it is the stuff you really want to compare and contrast with the blues--

Rags, Blues, and Jazz seem to have evolved separately, from the same roots, in the nineteenth century, and my thought is that those roots would have been a combination of the old spiritual music that Allen, Ware, and Garrison collected in "Slave Songs of the United States", and in the instrumental tradtion that turned into minstrel music--

The bottom line for all of this, to me, is that banjo drone string--it provides the steady beat that the syncopated melodies bounce off in everything from the old time rags on to blues and bluegrass--and the gospel singers did the same thing--traditionally using a long stick that one of the church elders would rap(!) while the congregation sang on the off and afterbeats, and later moving the part over to piano, drums, and funky bass--

Anyway, that swing between a straight steady bottom beat and a syncopated, improvised melody is at the root of the "American" music forms--rags, blues, jazz, gospel, rock and roll, rap, funk, swing, hip-hop, and whatever else--and, sorry to disappoint, it didn't come from Celtic Music---