As noted above, you can get black stainless steel fasteners, although they're not that common. Button cap bolts for light loads such as bottle cages look much better than standard socket caps. Carbolts on
ebay is one of the few retailers I have found. Some background from
www.albanycountyfasteners.com -
Black Oxide Fasteners made from stainless steel, (usually grades 18-8 or 304) have been chemically treated to alter certain qualities of the material, most notably the color. The treatment also adds a mild corrosion and abrasion resistance (in addition to the plain stainless steel’s nature), and reduces the now black oxide stainless steel fastener’s reflective properties. To achieve maximum corrosion and abrasion resistance, the black oxide may be wax coated or oiled. Wax coated product is more dull, where as oil sealed product is spun off to remove excess oil and may be bright or shiny. Wax coating complies with military specifications as a Class 4 finish. Black residue may come off of these fasteners, so they should be wiped down before being applied in applications such as jewelry or finishes where they may come in contact with hands.