Off of the top of my head, the only VTubers with brown avatars I can think of are Ironmouse (Puerto Rican) who has a few models with tan toggles, Sylphy (German voice actress who is (half-something?)), and Scarle (half-Mexican, half-Filipino). Unless there's some super popular Nigerian VTuber or something that I just wouldn't have crossed paths with, I genuinely don't think there is a single prominent VTuber with a black avatar, although obviously black VTubers exist. It's hard enough being black in the anime community (see: anime nerds flipping their shit whenever a black person dares post cosplay photos), but I'd guess it's even worse in the VTuber community. Plus there's the added difficulty of finding an artist who will actually make you a black avatar and not just a generic anime avatar with dark skin.