Hell, just parkerize it. You need phosphoric acid, distilled water, Manganese Dioxide?(the stuff in alkaline batteries, between the carbon core and the outer shell)
Dunk it in used Dexron ATF after parking. Plug the barrel BEFORE parking.
a fantastic hot blue can be achieved with nothing more than distilled water, lye, saltpeter(sodium OR potassium nitrate) and a heat source capable of maintaining a water temp of 285-305 degrees F for 30-60 minutes. Gives a deep black that is harder than the underlying steel. Don't even need to plug the barrel. It's wear and corrosion resistant.
All of you really should join the home gunsmithing forum. They have a gun and gunstock finish forum on there that applies just as much to air guns as it does to powder burners. After all, wood is wood. Metal is metal. When it comes to refinishing, it matters not whether it's propelled by air or by hot gas.
There's a dude over there who built a NICE 8 shot lever action .22 cal PCP rifle that Marlin and Browning would be PROUD to claim as their own. No Joke. He's also building a double barreled PCP rifle.