Alright, let's start on the ground. There is absolutely nothing wrong with black people in MTG. We've had them for decades. The problem is not Teferi, who has a character. The problem is not the Asians of Tarkir, not Koth of the Hamer, not the black people on Dominaria, nor the Indians on Kaladesh.
The problem is someone like Kaya, who was made first and foremost to be a black woman, actually, THE first black woman planeswalker, before a character. This is reflected in the fact that Wizards hired two diversity bloggers to make sure they didn't write another stereotypical independant strong black woman. If Kaya was a character first and black second, which should be the rule of thumb for every character made, they wouldn't have HAD to hire people to give them pointers on how to not write a bad black woman because she'd be her own character.
If designing for the character's race comes before designing the character's character, then you have a bad character who comes off as pandering.