From the Accurate Plating & Weaponry site...
"Black Chrome Facts
Black Chrome is the second hardest black finish available for firearms and the hardest finish available at a reasonable price. It is the premier finish when concealment is the first priority.
Black Chrome has many of the same properties as regular chrome. However, in changing the plated deposit from a Light Grey color to Jet Black, we diminish two major properties of regular chrome, hardness and abrasion resistance.
Black Chrome is slightly harder than Nickel. It was never intended to be a highly wear resistant finish, like Hard Chrome. It has much greater wear resistance than any form of Bluing (Black Oxide) and due to the presence of Nickel, or Chrome, as a base coat, a greater corrosion resistance than a single coating of Chrome, or Nickel the to the base metal.
Some of our clients are adamant about Black Finishes. This is the best bang for your dollar, if you are stuck on Black. For technical reasons we only offer this finish in a Matte surface texture and suggest the use of a wax impregnated oil on the exposed surface. Brownells offers a water soluble oil “Pro-Sheen†that is wax impregnated and works well on Black Chrome at two parts oil to one part water. "