In medieval Europe you had you had Mongols in the East, North Africans and Arabs in parts of Spain, and Turks in the Balkans. Beyond that you also had Arab traders in Italian ports and a delegation of Ethiopians that visited the Pope in Rome. So it isn't entirely true that the only people ever in Europe during the medieval period were white.
In any case, Thedas isn't medieval Europe. The Rivaini have skin tones that can range from tan to ebony, so there are black people living in Thedas. The main difference between Thedas and Europe is that Thedas seems to be in its world's southern hemisphere, with the northern regions of the continent having a more equatorial climate. Most of the darker skinned people might live in the north in that case, rather than the south.
I wasn't saying its exactly the same as Europe but generally it is. And generally white people are associated with Europe while other races are associated with other continents/areas/whatever.