I've used it on traps, but not on a gun. This was going on 30 years ago, when I ran a trapline, as a youngster. The stuff was called "logwood crystals", and it was just as the name implies. A crystallene substance, that was dark blue/black. You added it to boiling water, and submerged your lightly rusted traps in it for a couple of hours. I don't think it would make a very permanent blue job, on a polished surface.

Jeff