The Real McCoy,
I've just had this process done,it is an oxide coating that uses a secret process to 'plasma' coat stainless.An oxide is 'sprayed' with the plasma arc which turns the stainles black.
I can't go into more detail , because the company was very secretive about the process , that's all they would tell me , or my gunsmith.
This was done here in Australia , there's only 2 places that can do it here , but they're both US owned , so you probably have more options than I , over there.
I am impressed with how well it comes out , I would recommend it to anyone that's doing a custom stainless rifle.
By the way , they can also do a matte grey finish as well , it looks like sand blasting , but isn't!
Hope this helps!