Metacritic and Rottentomatoes have undergone score inflation in recent years, so I am not sure that is the method I would have gone with to pick the top 25 films by black directors.
The no film over $35M budget by black directors in 2015 stat is pretty crazy though.
I am having a hard time thinking of big budget films by black directors ever now. Fuqua had King Arthur in 2004 ($120M) and a bunch of mid-budget stuff. Singleton's biggest was Four Brothers (~100M). I don't think Spike Lee has ever worked with a major budget. Ditto for Tyler Perry.
Unless I am missing something major, Black Panther might be the most expensive project to be handled by a black director.
EDIT: We were having a conversation last week about how Latinos and Asians have even poorer representation in Hollywood as actors in major movie roles, but at least they are able to direct films with a budget beyond what Hollywood deems fitting for an "urban" film.
EDIT 2: Actually, thinking about Justin Lin and James Wan made me remember that F. Gary Gray is directing Fast 8 now. That will beat Black Panther to release, and will probably have a budget over $150M.