Years ago I heard cold blue could be used to touch up color-case hardening..at the time I took it with a grain of salt & forgot about it. Then I needed to touch up a small shiny spot and a small scratch on a vintage rifle..I remembered the cold blue advice. I tried Birchwood Casey..no good, I then tried Brownells Oxo-Blue..used a tiny bit on end of toothpick..rubbing in with Q Tip, both spot & scratch dissapered! I then coated with Break Free CLP & next day wiped off excess..and bluing with it!
I de-greased & tried the Oxo-Blue again..and again everything blended in..but this time, I used Hoppe's oil..this time bluing stayed put..and is still there after much handling and use..I never would have thought a type of oil would make a diffference.