Brass will blacken with a fresh solution of liver of sulphur (potassium sulphate) which should be easy enough to find. Make sure the lettering is chemically clean, and warm the brass to make the process go quicker. There are also proprietary patina chemicals like Birchwood Casey's "Brass Black" that's available in gun stores: Birchwood Casey Brass Black - Shooting Accessories - by Birchwood Casey - Fast-acting Liquid Used by