If you're looking for a gunsmith to do it, I'd check with Donald Franson (Honeybee is screenname here on OSA) of Franson Firearms here in Tulsa. He's a finish master, I'm just not sure if he's setup to do black oxide.
We have a plating shop where I work and we do zinc plating in clear, yellow, and now black. We also do electroless nickel. But, from some of the tests that we have done, we get a great finish with black zinc that holds up better than black oxide.
For anybody in the Norman, South OKC area I can do cold black oxide on small parts. If it is steel (not stainless) and will fit into my 2 gallon tank I can blacken it.
Black Oxide isn't a very durable finish, and you won't find too many shops that offer a true hot black oxide. The cold black oxides out there wear ever quicker. The oxidation occurs only on the surface and doesn't "penetrate" into the steel, rust resistance is minimal. But I do have to admit it does make a cool looking finish though. I prefer the look of Black Oxide over Parkerizing. I've used the kit from Caswell before and it works fine but doesn't last that long, be prepared to touch it up constantly. I found that a good paste wax (Johnson Paste Wax, Renaissance Wax, Howars, Lundmark, etc...) will help maintain the finish.
The high quality paint on finishes (cerakote, gunkote, molyresin, black-t, etc..) in matte black has pretty much made black oxide a dying option.