I mean, yes, there were the occasional black people in medieval Europe, but I'd expect them to be pretty rare in Central European nobility. Especially as Emperors.

Look, I'm not especially opposed to having black people in this, but having the offspring of the Emperor be black creates a ton of knock-on effects. And I don't think this was especially thought through, either. It feels like stunt casting for controversy, instead of something done for world building.