Never tried it but Dunlap in "Gunsmithing" says this:
To Blacken: Immerse until black in the following solution, then wash and dry, (use rubber gloves):
Nickel Ammonium Sulphate 525 grains
Potassium Sulphocyanide 3 oz.
Copper Carbonate 2 oz.
Water (Distilled) 1 gallon