I use LMF browning chemical and from what the instruction state when the parts are brown boil them in distilled water and they will turn blue black in color.I used LMF on my m/l it is a slow rust process and it turns out excellent
just read the instruction that come with the bottle.I here also www.midway.com also hasa slow rust blueing chemical they works excellent but can't remember the name ,there was an article in the american rifleman that i came across a while back