The original bridge is chrome, but all the accents on the bass are black, so we decided to blacken the brass. Investigation on rec.crafts.metalworking, the metalworking FAQ, and Deja News yielded two homebrew formulas and one commercial formula, sold by several different vendors. The commercial product uses selenic acid, copper chloride, and copper carbonate in denatured alcohol. It's sold under names such as Blacken-It and Brass Black. We didn't try this mixture.
Copper carbonate is used in pottery glazes, and is available from large ceramics suppliers.
It's a greenish powder that is insoluble in water, but dissolves readily in ammonia, turning the solution a striking royal blue. A teaspoon in 8 ounces of ammonia gives a saturated solution. Blacken-It, then buffing the rinsed surface back to a coppery brownish sheen. We buffed and sealed the surface with black shoe polish, then clear carnauba stick wax. And here's the finished product, mounted on a Univox bass from the late 1960s or early 1970s.