Let's See.
Last blockbuster Summer movie to make a splash was X-men with $232 million (per wiki) domestic box office. Average movie ticket price was 8.33 (according to this website
http://www.hollywoodreporter.com/news/average-movie-ticket-price-hits-720316)
232,000,000/8.33= about 27,850,000 million movie goers. So about 28 million out of the the 300 million Americans watch Movies. 28/300 = Call it 10%. So 10% of all Americans take the trouble to watch movies.
Let's Apply this to the Races.
white 77% (includes hispanic)
Black 13.2%
Asian 5.3%
Native Indian 1.2%
Hawaiian/ Pac Islander 0.2%
Out of 300 million Americans that's:
white 231m (includes hispanic)
Black 39.6M
Asian 15.6m
Native Indian 3.6m
Hawaiian/ Pac Islander .69M
Using 10% of each as movie goers. We have
White 23.1 million
Black 3.96m
Asian 1.56m
Native Indian .36 m
Pac Islanders .07m
If we assume that only Black people watch Black Movies, which is mostly true from my understanding, then the likely amount of money that a Black movie could make (assuming no repeat viewings) would be 3.96m* 8.33$ or 32.98 million dollars.
That is just a normal movie. Certain films that can galvanize the the Black Community like Tyler Perry's films would probably have a better Turn-out percentage.