You wouldn't say, "a two armed person pulled a knife on me!" because two arms is the norm, outside of, perhaps, prosthetic clinics. So, I assume that "black" and "girl" are perceived as exceptional enough by the speaker to warrant mention.

Personally, I'm more concerned with the tone and context. My extended family insists they aren't racist, despite spitting out the word 'black' as if it were stinging their tongue. Also, they like to casually mentioning how much they dislike the "beady yellow eyes" of Indians (maybe), and integrate the n-word regularly into their humor.