There are two primary types of Rust Blue, Slow and Express Rust Blue. The end result of both of these methods are the same.
With “Slow Rust Bluing” a chemical is applied after all metal preparation is done and the firearm is placed in a slow rust box to build up a layer of rust on the outside. It is then carded off and polished.
With “Express Rust Bluing” the firearm (again after complete preparation is complete) is heated and the Express Blue is carefully wiped onto the steel. It is then boiled and carded. Both of these processes are repeated until proper color and coverage had reached it’s desire level. You can see some of the process in the pictures below.