The "old process" which gave a gloss black color is no longer done by Benjamin/Sheridan/Crossman/whoever was acquired this week. They stopped doing that process back in the mid 80's, IIRC.
The "new process" only results in the matte black finish seen on modern Benjamin/Sheridans.
Tim MacMurray from MacMurray and Sons can probably hook you up with the closest possible finish. Google "Mac1" I think...
Most of the "Gnome-Kotes" use heat, and as Sheridans are soft-soldered together, I'd be hesitant to try, or at least try on a wreck first.
If you had the gun stripped down to individual components any refinishing process that's compatable with the brass construction and solf solder construction could work. Their stain nickeled guns looked pretty nice.
I had a few paintball guns powder-coated back when I was airsmithing in the 90's, but they were billet aluminum. TOUGH thick finish. I did a WGP Sniper in John-Deere green just for giggles...