[Scorch]

I once tried rust bluing a shotgun by degreasing with ammonia and spraying it with distilled water and closing it in a cabinet, then boiling and carding it (the results were less than perfect, but it worked). There are many ways of inducing the surface rust: fuming sulfuric or concentrated nitric acid (as mentioned by Unclenick), ammonia, copper sulfate (many of the commercial rust bluing sloutions are copper sulfate), and silver cyanate all cause the fine surface rust that can be boiled then carded off and leave the black surface oxide. The rust/boil/card process has to be done several times before the bluing is even enough to be called finished.

You can also use Brownells' Dicropan to "rust blue" metal. You use it the same way, but in addition to the rusting process, you get some of the staining from the cold blue that can even out the finish.