Rust bluing is a process that is actually fairly old. It involves polishing the parts to a desired finish, then applying a weak acid solution to evenly rust the parts in a very thin layer. The parts are then either boiled or steamed until the rust is converted into a more stable oxidation state. After cooling the majority of the black iron oxide is removed by either a very fine stainless brush, or steel wool.
This process is somewhat labor intensive, however I am quite pleased with the results thus far. I will probably use this on many projects to come.
For those curious, this is converting Fe2O3 into Fe3O4.
Please note, in this first picture I overlapped too much causing it to "copper" which is not desirable, it still looks cool. 2nd item is a part for my lathe that I machined to hold large items on both sides of the head stock.