While browsing through the stuff at Dayton, I spotted a keyer paddle for sale with a particularly nice dark brown finish on the base. It didn't appear painted, so I asked the vendor what kind of finish it was. He told me the base was made of solid brass, and the finish was done with a product called Brass Black.
It is claimed to put a durable, stable patina on unpainted brass, preventing further oxidation. It looks like it might make a good finish for brass parts like knobs, dials, panels, door hardware, etc, which tend to quickly tarnish upon contact with moisture or oil from finger prints.
I found several sources on the web. Van Dyke's Restorers and other vendors sell 3 oz. bottles for about $10. One gunsmithing supply outlet has it for $7.31 a bottle.http://www.vandykes.com/product/brass-blackhttp://www.cheaperthandirt.com/23451-1.html
Similar products are listed for other specific metals, such as copper and aluminium.