Take Journey to the Centre of the TARDIS as an example. Yay, we have 3 black brothers running their own business. But can the actors act? Hell no, they were awful. They weren't awful because they were black, they were awful because they were bad actors.
Not the first time Doctor Who has had that problem.
A black actor was cast in role in the Pertwee story The Mutants and while its not a well regarded story, the abysmal performance of the actor in question doesn't pass without comment (though more for what's on the screen that actor's ethnicity).