The color black is often used to describe feelings. A “black” or “dark” feeling is one that makes a person feel completely depressed, hated, offended, or as if they’ve been put beneath a cloud. It can also describe a very mean insult.
So in that moment, Carter called Declan “a pretty cool dude”. It was a compliment, because it meant Carter liked Declan, but it was a “black insult”, because Declan does not identify as a boy, or a “dude”.
So it was both, at the same time.
Does that make sense?