I think they're less common in historical than in paranormal, but there is a thriving market for historical African-American romance which runs the gamut from "obstacle to romance is racism" to "in our peaceful prairie town settled exclusively by African-Americans, we never encounter racism and have the exact same storylines any romantic couple would have."

ETA: That's my point: you'd come across more diversity if you read more self-pubbed books. That's where the diversity is.