Hey, John: No, singing the same note an octave apart is fine in black gospel, and doo wop, too. How it sounds depends on how many voices you have. If you have a trio, and the two harmonies are an octave apart, there's way too much empty space between them. It's not that harmony always has to be in thirds, but if you are looking for a smooth blend, I think that too much distance between harmony lines sounds much worse than occasionally sharing a note.

The two groups I'm talking about have a very different mix of voices, and they determine the harmony to a great extent.

The Sentinels are a five man group. They have three tenors, a baritone and the bass singer, whose range is much closer to baritone.
Because they are tenor-heavy, they sing songs in a high range so that the bass line could even be sung by a tenor.
The Gospel Messengers have two baritones (both of whom can comfortably sing bass) a bass singer, and a second tenor (the lower range of the two tenors.) Harmonies are clustered more toward the low end of the range, just because of the voices.
When I sing baritone with the Sentinels, I am pushing the top of my range. When I sing baritone with the Messengers, I am hovering near the bass harmony. In both groups, the harmonies are close, but in a different range. In the Messengers, Frankie sometimes sings in falsetto, an octave above me. When we have a tenor singing harmony between us, it sounds fine. When we don't it sounds empty.

Jerry