I just depends entirely on how and where the fantasy is based. For English stuff yeah there weren't that many black people and especially weren't many aristocrats that were black (as all fantasy pretty much follows the aristocrats only). But say in France the situation was different, I can't think of any medieval famous black people from there but there were plenty once the age of sail and empires started. It really depends on the locations the myths are from.

Like a guy below says tho if you're doing the setting in the caucuses or even Italy and the lower Mediterranean then yes there were plenty African peoples living there.

It's a question of nuance and where the mythology of the story is based on. Not unlike how it's shitty to include a white main character in a story based on myths from somewhere in asia.

However if a person is making up a new story not tied hard to mythology they can obviously do at they will. Plus they can follow people who are lower on the social class level where there were people of many mixed ancestry have existed. The thing is they were always rare and thanks to history from that era being at least inherently racist they get left out from whatever accomplishments they achieve.