I did the lockplate, butplate, screws and trigger of one my SMRs a little differently. I heated the parts in charcoal to around 900* and quenched them in canola oil. This makes a deep blue black. It is not burned on oil. IT will not be perfectly even on large parts. It works really well on screws, I do all my screws this way. My intention was to not make a perfect job but to look like something that might have been done by a blacksmith.
To test the process, polish a piece of steel. Heat past the colors, to just faintly glowing red in shop light, then dunk in canola oil.