Nationality doesn't determine whether someone's black
This. Even in the real world, Black encompasses a whole host of ethnicities and a wide range of ancestries. Around the world, it is associated with a different groups, but it's not exclusively descriptive of any one of them. Real or fake, Kat being Black isn't really a question.
Intended associations and the character development process are irrelevant here.
I'm gonna put a call into myth busters so we can have this one and done once and for all.
She's NOT "Japanese" and she's NOT "caucasian" and she's NOT "french" either, so...bust those, as well. Thx. --- "As God's chosen people, clothe yourselves in compassion, kindness, humility, gentleness, patience...Forgive others...And...LOVE!" Colossians 3:12-14