You can try contacting a company called Pioneer Metal Finishing. They do all kinds of different anodizing and plating. I use them a lot at work. Another place to try is called Professional Plating in Brillion, WI. We use them for electroless nickel plating, but I know they do other stuff too. I have no idea if either have minimum orders, but it might be worth a call.
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Thanks I will have a look.

I know it will be a more expensive process than black oxide, but the quality is so much nicer and it doesn't rub off. I have to figure out what I think it would be worth per custom for that service though... adding up to $50 per SAK for a high end one would be doable but beyond that I think it would probably be too much. Right now BOing a SAK springs and parts runs me about $20-$25.
My local guy has the knowledge, but not the setup yet. Setup is simple enough, high vac, argon, negatively charge part and sputter away. The particles hit the metal with such velocity they embed right into the steel. But dialing in the right parameters are what the trick will be.