Yeah, it was racist. There are degrees of racism, like everything else. It's not cross-burning racism, but referring to her as "black" is putting her in a category that is irrelevant to the subject. And that's an -ism.
It's human nature to divide people into "us" and "them". It's also human nature to be able to change. A lot of racism, and stereotyping in general, is unconscious. It's something that was picked up and is practiced without realizing it. I am now appalled at things that I have said or done when I was (much) younger, without realizing it at the time. But I learned and still am learning. Often because it was pointed out to me. Perhaps that is/will-be the case with Sunworshipper.
Why should one listen for it? Is old-or-young, white-or-black significant? "Us or them" is natural, but we should be aware of when it's happening and be careful about how we use it. E.g., not referring to someone as black when it's irrelevant and in a negative context ("Weird requests").
Bob