From what I get from Rowling, we are supposed to think that in the Potterverse the discrimination that matters is about how much wizarding blood people have and between magical humans and other kinds of magical beings, and that other kinds of discrimination that we see in our world don't exist there. So she has non-Caucasions sprinkled in the background (heck, Kingsley gets to be Minister, doesn't he?). We are supposed to see that neither Ginny nor Harry are racist because they date non-Caucasians, but they can't stay with them because Rowling already decided they belong with one another. She even has the token black Slytherin (but he is mentioned so briefly I totally missed his skin tone). Maybe this aspect of her message would have been clearer if Draco had married a non-Caucasian, but she needed Scorpius to be a replica of his father. Heck the Blacks could have had some black women married into them, that would have been consistent with Rowling's style. Sirius the black Black would have worked just fine.

- Oryx