That'd not going to come off. Because that's just a spot where the blueing has rubbed off. If it really, really bothered you, you could reblue it. I'd just leave it.
Like SG said. It does bother me, so I keep bottles of acetone and Brownell's Oxpho Blue on hand. The fix takes 2-3 minutes. The rub on the edge may be caused by your holster. If so it will return.
They make touch up pens that may do it. https://m.basspro.com/Birchwood-Casey-Super-Black-Touchup-Pens/product/34241/
A black permanent marker sometimes covers small spots. VT How well does the Oxpho Blue or the touch up pen work? Does acetone remove any of the other bluing? Would rubbing alcohol be safer or worse to use than acetone? You are just trying to remove any oil or residues correct?
The Oxpho Blue works fine; you only need to follow the directions - without trying shortcuts. It penetrates deeper than a permanent marker (which of course doesn't penetrate signicicantly). I've not used a 'touch-up' pen. Acetone (aka fingernail polish remover) removes oils not bluing, and evaporates very quickly without residue. It is not flammable, and the fumes disperse rapidly, nor are they poisonous. If the rubbing alcohol is really rubbing alcohol, it contains a small amount of an oily substance, e.g. glycerine -which leaves a residue. I get the impression that nowadays some so-called rubbing alcohol is just isopropyl alcohol and water; that works pretty good for our purpose; for rubbing not so much.
