Use Brownell's Alumanhyde, available on their web site for about 7 bucks a can, plus shipping of around 8 bucks (so order more than one can! LOL) They have it in Semi-Gloss Black and Matte Black...(they also have it in other colors too, OD green, Parkerized Grey, Sand, etc) Both semi and matte black looks REAL good; even better than some original finishes I have seen. The Matte Black almost looks like a VERY dark grey parkerizing finish... It is impervious to most all solvents and bore cleaners. It will scratch, but hey! so will blueing...and is as hard as any paint I have ever seen if used according to instructions and allow to cure for at least 3-4 days after painting and before putting everything back together. (note: be sure to pay attention to temperature stated on the can, 70-90 degree application temperature...) It also mentions that to shorten the cure time to let it sit or hang in the hot daytime sun for one day after application. I tried that on my last one and it was cured in two days...one day in my shop and one day on the deck in the sun.
Best wishes,
see ya,
T2