Yeah, the Roman Empire seems like as good a place as any. Of course, "Black" was hardly a category in that period, but Mauretania took its name from the Greek "μαῦρος" which meant "black" or "dark." So...an emperor hailing from Mauretania would logically be black and on the throne well in time. Maybe butterfly one in during the Military Anarchy, it was kind of a crazy anything-goes period.
A Mauretanian emperor would likely be Berber, more than Black that were a bit further in the south.