All blued parts still require oiling to prevent rust. Bluing, being a chemical conversion coating, is not as robust against wear and corrosion resistance as plated coatings, and is typically no thicker than 2.5 micrometres (0.0001 inches).

Is bluing a form of rust?

Ironically enough, one of the most common firearms finishes, bluing, results from a specific form of steel oxidation – that is, rust. Instead of the red-brown color associated with rust (Fe₂O₃), bluing forms the blue/black oxide Fe₃O₄.

Does cold bluing protect metal?

Bluing is controlled rusting in a dark color; it's the red kind that causes an uproar . So, cold blue does protect the metal to an extent, from the red stuff.