so that "brass black" stuff can be bought from any jewelry supplier of chemical supply house as well. They use it to add a patina to brass and make it black or antique. If you have any liver of sulphur lying around you can use that as well. Use common sense when you use it, glasses, ventalation, gloves...
here are some brass patina formulas:
Patina Formulas for Brass, Bronze and Copper