Might be hypermelanization from being stung by an anemone. The area of the black marking makes sense as well. Could also be swipe mark from another tang. Any aggression going on between fish?
Excellent advice by @Tahoe61 above! A coral sting is another possibility, and do keep an eye on the two tangs. They are both Zebrasoma genus, so skirmishes between them are likely sooner or later.