Bluing
blu·ing ˈblo͞oiNG/
noun
  1. The chemical process of artificial oxidation (rusting) applied to gun parts so that the metal attains a dark blue or nearly black appearance.
  2. A controlled chemical rust process that produces a very dark, almost black, blue finish to the steel parts of a firearm which enhances the appearance and provides some protection from unwanted rust. Sometimes it can have a slight brownish undertone. The percentage of blue finish remaining on a gun can be a proxy for describing its condition.