I expect they knew better than to try offending people of African descent in the makeup department - this wasn't that long after Laurence Olivier made that mistake.

It should be pointed out that the objection of the English actors' union to Jones was not that he was black, but that he was American. (Or, to put it perhaps more cynically, that he wasn't a member of the English actors' union.) 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.