Jerry, I don't know much about the connection/derivation of black Gospel from spirituals, maybe there are some insights to be gained from someone who does.

But listening to the Georgia Sea Island singers, for example, I get the idea that the black singing tradition during slavery was very likely to include some kind of percussion, even if it was hand clapping or "hambone music" played by slapping out rhythms on one's own body.

I still think of them as "unaccompanied singers" though. Oh well.

PoppaGator, I loved your comments on work songs and the fact that they were sung in rhythms that were determined by the work. It's maddening sometimes to hear shanties or work songs sung out of rhythm or sung way too fast for anyone to work to. The songs lose so much that way.

I once heard Peggy Seeger say that the Revival loses a lot when singers leave out the traditional ornamentation. She said, "They don't realize that the ornamentation is part of the song." I often feel that revival singers "homogenize" their singing. It loses all the traditional stylistic ornaments and subtleties. Singing unaccompanied give you a chance to listen to, and try to control, the subtleties. If you're playing an instrument, or even accompanied by one, it's harder to pay close attention to using your voice as an instrument.

It also does make it harder to stay on pitch. I once phoned a professional singer, a friend of a friend, looking for the verse to "Glocca Morra." She couldn't get her phone close to her piano and had to sing it unaccompanied and she could NOT stay on pitch. That made me realize how much of a challenge it is to sing unaccompanied, and how much of an accomplishment it is to do it well.