Perhaps it should be. But as it turns out, there are plenty of excellent black metal bands with christian lyrics. Antestor, Horde, Crimson Moonlight, and Extol sometimes, to name a few. And yeah, I don't really understand christian lyrics in metal that well, it sort of goes against the aesthetics of the music in my opinion, but some bands can actually pretty awesome lyrics.
Christian black can be pretty cool but the term "unblack metal" is stupid.