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Something like this; GunBlack.com - Black Oxide and Manganese Phosphate
It's used a lot on machine parts, nuts n bolts and such.

Sorry, I know they exist but not where to get them.
I've used it on military guns, we had 2 bottles, one for cleaning and the other with the phosphate, it was really easy.

Then if you want it really black there's a lot of phosphate solutions to try.