Dunno, that's a new one on me...but, FWIW, several of mine have worn bluing on the face of the barrel breech, I think that's normal for one that locks up tight. Every time it closes, it's gonna be rubbing against the standing breech, at least on the lower part. As long as it's shooting good, I wouldn't worry about it, just use a good protective gun oil on it, or if you don't like the looks of it, degrease the barrel and use some Birchwood Casey Perma Blue on it to reblue the worn spot.