Oddly enough, the Harry Potter franchise handled this well.

Her ethnicity should be Innsmouth.

Trace her back to her origins, and she's literally based on a Danish folktale. I can guarantee you no one in Denmark when the story first was told was thinking of her as black.

But then I think all of those examples were bad and should never have been cast that way. A black Anne Boleyn is exactly as bad a choice as a white Mansa Musa, for example.