I think his relationship with reality is not normal.
Most of us recognize that there is an external reality that we learn about, discover, explore, etc.; and we all have a perception and understanding of reality that we're constantly updating based on feedback.
I think Donnie believes reality is what he thinks it is. If he's not aware of something, it doesn't exist; if he learns something new, nobody knew it until he learned it.
So at some point he thought Kamala was just an Indian-American, then later he learned she was also black. So he thinks reality changed, or she changed, and "turned ba-lack".
Couldn't be that something existed that Donnie didn't know. He must have been right that she wasn't black, so if she is now, it's because she became black.
A normal person would not have said she "turned black". A racist but otherwise normal person might have said something like, "she never talked about being black before, then she runs for president and all of a sudden, she's talking about being black". But the concept of "turning black" would not have even occurred to a normal person.