Wonder if this phenomena is related to the "Big Chief" culture in New Orleans. It seemed the African transplants to those areas describe themselves in ways that seem more like Native American to the uninitiated.
I'm afraid that's not it, but rather (probably for the reasons I outlined above), people who crossed the race line made up fake ancestry to explain their "different" appearance without admitting to African heritage. I know some families also claimed to have Middle Eastern or Italian heritage to explain their dark features. It's sad, but they did what they felt they had to do to preserve their safety and standing in the community.