I've always felt that r&b acts basically did everything from doo wop, soul, disco, funk, yauht-rock, boogie, electro-funk, smooth jazz, new jack swing, & pop, but record labels never considered their music in the same category as their white counterparts who did soft rock, blue-eyed soul, shoegaze, new wave, etc.

It's tons of r&b artists who had guitars in their music or had rock elements in their sound but they still were considered r&b and soul artists regardless.

Once in a while you will find artists like Michael Jackson, Sade, Prince, Aaron Neville, Mother's Finest, Seal, Terence Trent D'Arby, & Meshell Ndgeocello, either in rock or r&b section of a record store and sometimes in both depending on the management.