CARBON STEEL BLUING
The process of Bluing is a controlled oxidization (rust) of the metal. It’s not going to work on stainless steel or other metals besides carbon steel for that reason. This thin layer of rust (which is a blue/black color thanks to chemicals in the bluing solution) and the fact that the oxidization is controlled forms a protective layer over the steel beneath it. Basically the rust on the outside keeps oxygen from getting at the steel on the inside and allowing it to rust.