Originally Posted by DCWilson

The "black stuff" problem worries me a bit, but it shouldn't be there and I would do what is necessary to get it off. Try some straight acetone and see if that dissolves it. If it stays tight, rub it with the side of a brass cartridge case and see if that takes it down. (The point being that brass is softer than steel, and the radial surface of the case has no sharp edges that might inadvertently scratch the blue further.)

Worst case, it might be corrosion bubbling up through the blue. If so, I would get it off and just use a drop of cold blue to cover the resulting mark.