They'd have been fine with black Englishman, but apparently there wasn't one available - or, at least, none that the director felt was up to the role.

Oh, I understand that...

But that just underlines a different sort of racism. Such asL How hard did they actually look for a black actor? And/or why are there so few blacks in the British Actors Union (this goes back, too, the perrenial DW comment about how all Timelords are white ;-)