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.
If wiped down once in a while the bluing will last many many years. Bluing will ahow holster wear on a working gun. (police officer, security guard, someone who trains a lot)
Bluing can be removed with polishing. A nickle plated, or chromed gun must be stripped before they can be blued.