Theory: Jazz is Inuit
I know it sounds strange, but I don't believe Jazz could be black, and it's not for the reason you might be thinking.
Ueda's art doesn't follow a very common style in Japanese art, which is one of the reasons we didn't like the art style of the new game: Mukokuseki. It is the intentional choice not to have a prominent ethnicity in a character so that more people can identify with them. Ueda doesn't follow that style in his art, making the ethnicity of his characters quite obvious; Jazz has more Asian features compared to other characters in the game
I have three theories about their ethnicity:
South Asian, being descended from a tribe like the Inuit (like the water tribe from Avatar)
She's actually tanned, it's quite possible, strangely they've never made a Gyaru character that has origins in Japanese rebellion.
She is Blasian
If you think I'm wrong, that's fine, I don't get paid for it lol, it doesn't even influence the game's story.