Mostly it is what the manufacturer wants to use. There is a plain old chemical blue found on most traditional styles of m/l and many inlines as well. Then there is a higher gloss version of the same type of bluing found on most sporting rifles like model 70's and such.I think it is called hot bluing. Then there is a finish I am not sure how it is done but is very high gloss and almost looks like black porcelin. I checked out a couple rifles made with that finish at Fox Ridge Outfitters a couple days ago. It looks very durable but I can imagine trying to do small touch ups would be tough.

If you wanted to duplicate any type of finish I would ask your local gunsmith about it.