So from a realistic stand point, there aren't going to be blacks in Northern Europe Medieval land and from an in-universe standpoint, they just have different races.
Just to clarify, I definitely don't think that it's weird for a game not to deal with race issues. I only think it's odd for a game not to depict people of color (to the extent that people are represented at all in the game), and I think it's odd that so many games make that choice. Depicting or representing as much of your potential customer base as possible has many benefits and virtually no opportunity cost.
I guess my whole thought on it is that perfect fidelity to either the history or mythology of the people living in early Medieval Poland is neither possible nor desirable, and I don't really understand what the opportunity cost is. As in, I don't see what difference it would make to break from reality in that way.
1 Fantasy stories already lose complete fidelity with history (there aren't going to be any elves, dwarves, dragons, or witchers in Slavic Medieval Europe), and Witcher as a series already doesn't maintain much fidelity with either Slavic history or Slavic mythology.
1It's unlikely that there were no people of color in Medieval Slavic Europe. The Slavic states were trading as far away as modern-day Serbia as far back as 5000 BC (link), and by the tenth century there was a robust trade network between the Slavs and the Islamic world (link). This network included both Islamic and Slavic slaves, and Islamic slave ships would sail as far as the Baltic Sea to acquire Slavic slaves. Muslims, Mongols, North Africans, and Spanish probably weren't uncommon inhabitants of Medieval Europe.