"All Black Cast"? Give me a break...
People referring to this as a "black movie" are ridiculous. The very fact that it's even considered to be one just highlights the racist culture of Hollywood and the underrepresentation of black people.
Take a look at the movie poster. 7 actors - 4 black, 3 white. First off, this is nearly a 50/50 split. Secondly, if you were to even reverse this ratio (3 black, 4 white) people would probably still call this a "black movie" even though the black cast is underrepresented. Is the original considered a "white movie"? No, that would be seen as entirely ridiculous.
This just goes to show how it is still such a shocking/unusual thing to have a healthy black presence in a movie that isn't "Boyz in the Hood", "12 Years a Slave" or "Dear White People".
Hollywood can't handle more than one or two black supporting actors in a movie that isn't about slavery, the hood, or the theme of being a black American without looking at the movie and thinking "well, that's a bit weird, isn't it? There's a few too many black people in there. Why isn't it a movie about slavery or shooting people?"
Hollywood and its audiences still racist as ever.