It’s really a simple principle, if you want black people to be a part of your theater, ask them to be a part of your theater. I’m not being glib. If your audience comes to a show and they only see young, straight, white males on stage, and they aren’t a young, straight, white male, they are less likely to sign up for classes or sign up for auditions. Hell, they are less likely to come back to see another show.
Edit: Some interesting discussion on the subject at Reddit.