I agree on this with you. If the voice doesn't sound believable, then rather have a voice actor who sounds right for the character being portrayed.Of course not but to me this is not a big deal if they want to use black actors for black characters. I say go for it. I've watched shows and played games where I wished they used black actors instead of very obvious white ones.
I'm not against ethnic actors portraying ethnic characters. I'm against their reasoning that only actors of a particular race or ethnicity, has to be a voice actor for a cartoon character of said ethnicity or race group.