Has your friend never carded anyone before? Do they not know how upset people typically get over being carded? The way you've told it, it sounds like you think you know more about your reader's job than she does.

If your reader was making the statement that these three girls were anomalously angry, then your answer is a complete non-answer, because it doesn't address the source of the anomaly at all.

Some possible answers might include

(1) It was just happenstance that the girls who were especially angry also happened to be black--could have been anyone.

(2) Non-black girls surely got equally angry, but this time you just percieved the anger as greater because of your preconceptions about what black girls should be like.

(3) The girls believed that the only reason you were carding them was because they were black, which they perceived as unfair (and maybe you were, and maybe it was).

etc.

But "they were embarrassed" doesn't cut it as an explanation for why they were so angry. Only for why they were normal-angry, within the usual range.