If "basic black" will do, here's an historic recipe. This was used to mark silent/suppressed/gallery 7.62x54R loads for mosin nagant. It will not dammage brass for reloading. The recipe is from part 3 of the link below-
http://guns.connect.fi/gow/arcane1.html
The chemicals are available from The Photographer's Formulary and your local grocery store.A recipe of the Brass Blackening Mixture:
Mix in the enamelled, stone-ware or stainless steel kettle:
2 parts by weight COPPER SULPHATE (Copper vitriol)
2 p.b.w. SODIUM THIOSULPHATE
1 p.b.w. WINE STONE (Cream of tartar; Potassium bi-tartrate)
40 p.b.w. SWEET WATER (preferably distilled).
Heat the mixture boiling. Add the cases. Cook them until the color is glossy black through the colors: rose-red > blue > bluish black. Cases must be carefully degreased before blackening: No "master's fingerprints" are allowed ! Chemicals used are not the strong poisons, but the mixture is not suitable for seasoning of the celebration punch: It may cause a condition called as the "hyper-emesis", when used internally: "per os" !
http://www.photoformulary.com/Deskto...ID=18&langID=0
Have fun-