I believe what you are talking about is "bluing" - or the process of chemically treating steel from rust. Essentially it's an easy, and somewhat cheap, weapons treatment.
Information on Bluing
If you are shopping the used market, and the ad says bluing is 60%, that means that the finish of your weapon is pretty mucked up. However, the nice thing about bluing is that, with some work to strip the gun down, and the right equipment, you can re-do the process and get a nice, almost perfectly clean, blue-black finish again.