The process of bluing steel creates a micro finish on the steel surface to prevent corrosion and rust. Putting this type of finish is very popular with guns as the process improves the surface finish, protects it and reduces the glare off the metal. While not nearly as durable as plating or painting, bluing is an inexpensive and simple surface finish.
There are a few different ways to blue a surface but all involve creating a very thin ‘coating’ of iron oxide on the surface. This coating prevents oxygen from further attacking the steel. This is similar to the way the green ‘coating’ (copper oxide) coating on the Statue of Liberty protects the underlying copper. Bluing gets its name from the coloring of the iron oxide (magnetite) the process creates. This is a different chemical compound than the reddish color typically associated with rust (also iron oxide).