One of the toughest black finishes I've ever gotten was one done by
heating nickel bearing stainless steel alloys. It's not real dark,
but a fairly dark gray color, very even too. My thinking is that it
was that famous passive nickel oxide we all have had so much trouble
with. I'm just suggesting, since nickel plating is a tough coating,
maybe a heavy nickel plate which is subsequently oxidized with heat
would be worth looking into.
David L. Huffman