As I said on another thread, as far as I can tell, it's not supposed to be an accurate historical drama, but a fictitious murder mystery set in that age. The characters are representations of their real life selves, not historically accurate ones. The difference between this and a white Mandela is that Mandela's race was a major part of who he was, Anne Boleyn's wasn't. There are many examples of white actors playing historically non-white roles. John Wayne as Genghis Khan springs to mind.
Having said all that, I haven't watched the show, nor will I. It's not my type of thing.