Well there are variations on rust bluing. Angier's book on finishes is a good source of info, and I did post an old recipe and proceedure for British military guns once. Usually you treat the metal, and then set it in a sweat box or the like. This causes a finish to form. Then you card/scrape some of the rust off, and do it again (sometimes re boiling the metal, sometimes not), sometimes 4 or more times. Its an involved process, and can take a week from start to completion. I have seen recipes that call for boiling in a solution, others that call for dipping in a solution etc. It is in effect though, the building up of a controlled oxidation. It leaves you a much more durable and thicker finish....usually.
For purely cosmetic issues I have found some of the cold blues will do a good job on some of the older metals, and in somecases a repeated application will do a good job, but it isn't as durable as any of the slower methods, or a good tank job.