Bluing is not a coating... It's a finish. It's a form of oxidation. the metal is soaked in a tank filled with water, and salt peter. Depending on the ratio of water to salt peter, the temerature, and how long the parts soak will determine the color and quality of the blue job.
There you go, some one finally stated it clearly.
Bluing is not a coating. It's a finish treatment.
...a controlled oxidation process, aka 'rust'.