Miss Molly,
Having been out of the game for awhile, as a provider I read your post and just wanted to give you a quick heads up on the Latino/Hispanic race response.
Dominicans, Puerto Ricans, Cubans, Brazilians, Venezuelan, Colombians, Panamanians and Hondurans (and mmaannnyy more) can have African features. I had/have a your exact same preference in reverse but didn't want to come across as a self-hating white girl
If race is that huge an issue for you, you need to notify the general public. Nothing wrong with backing up how you feel. I bet the men would much rather stay at work/home/a bar than run the chance of them not qualifying and waiting around. As much as you hate it when a man NCNS they feel the same when it's done to them (via a different means).
No one but you is running your business and if you aren't comfortable with something/someone, find a way to communicate it effectively AND respectfully. You've gotten some good suggestions, being very clear and concise about your criteria in your ad. Good luck gal.