This is what I have been doing the past week. It's an old form of bluing called rust bluing. In the olden days they used urine, but these days they make chemicals... come to think of it though, it is
Basically you take the bare metal and clean it very well. It can have no oils on it. Then you wipe the chemical on, put the parts in a humidity box and let the rust form. After the rust has formed you boil the parts and convert the reddish color to black, then polish off the scale with 0000 steel wool. I'm doing my 7th application right now. From what I have read I will reach a point where the parts will stop darkening. Then I am done.
This used to be done a long time ago, but because it was so time consuming other methods were developed. It is still considered the best looking form a bluing and is the most durable.
And of course it can be done to anything metal, knife blades, wrought iron, anything you want to help protect and turn the color a bluish black.