I'm into model railroads as well as photography and there is a solution called Hobby Black #'s 1&2.It contains Selenium Dioxide,Nitric Acid and Methyl Alcohol.All you do is immerse the part/parts into this solution and in less than 2 minutes you have a pitch black matte surface that will last quite a long time.It works on nickel,brass,steel,copper etc.You can probably find it at any hobby store that carries model train materials or just model materials.

Doug