To be honest, I kind of like the approach of "Well, the history of this country is white, and you have to think about how a black character would have gotten here if you want to play one here." more than I like the approach of "Yeah play whatever characters you want! Black or white! A black person applying into a family of all white people? All great! Skin color doesn't matter in genetics at all!"
Why? One is just the thematic recognition of race as a thing that exists and is limited by genetics but somewhat excluded due to the limited geography of the game, and the other is color blindness.
We're past the time that colorblindness is considered socially forward, aren't we?
Diversity is powerful because everyone is different, people with different colored skin might come from different places, have different experiences, and have different cultures that lend them different strengths and weaknesses. This is what makes the vast range of humanity to be as beautiful as it is. Speaking as a POC I am always more irritated by seeing race as a factor completely ignored in some dumb nonsensical way than I am to seeing people be sticklers about what race is supposed to inhabit what geographic location in some fantasy world. The absolute worst thing though is villifying a race from a location (ex: Southrons in LOTR).
I did play in a game once where the staff was very dramatic about how you weren't allowed to have red hair or blue eyes (or anything but brown hair and brown eyes) if you were applying as a certain race, and the constant harping about all this did annoy a lot of people. Nobody called it racist, but that's because the power dynamic doesn't exist.
Nymeria should realize that if she doesn't expand the geographical borders of her game world, then people are totally within reason to call her racist for thematically excluding a color of skin that is oppressed in the world. At the very least, Nymeria should realize that she is creating a haven for actual racists to not have to encounter black characters in, or to always be in a position of power over black characters.
Authors and game runners in general should wake up as society wakes up to injustices, and allow developments to occur in their fantasy worlds that makes things less racist. It'd be actually great if something could happen like an embassy of black people come to King's Landing, get trashed for their skin color, and then you make a whole plot conflict over it where the racist bad guys lose in humiliating ways and then a great trade empire is established with people of many races and cultures. That way you acnowledge disparity, let people play out righteous fights against injustice, validate what's right and fair, and now have a world that isn't racist at all while still recognizing diversity in the best ways.