Jim,
H&M Metal Processing regularly does Black Nitride treatment for individuals. Last I checked, they charge a flat $200.00 per single load of parts, without being too fussy about how many guns contributed parts to that load (within reason). Steel parts only; no springs or soft metals (brass or solder will be melted). Go to their website to learn the details of the process and its benefits compared to other finishes (weather proofing and wear resistance). Be prepared to be impressed.
Steve