Period color-case hardening has a blue-grey 'smoky' look to it - like what's seen on a Colt Single Action Army.
Over time - it faded due to use and sun - hence the phrase - 'traces of case hardening in protected areas' - so beloved by dealers in old firearms.
Modern guns often have either an applied wax, varnish or other 'clear' type of sealant to keep that from happening, and removing it's pretty simple - without any 'mechanical' assistance.
Just use a cotton swab and a Toluol-based solvent, and you should be set - and even nail polish remover will work well.
Remember to keep the surface lightly oiled, as you would the rest of the piece.
Good Luck.
Vaya,
Scouts Out!