Keep a light coat of oil on the metal. There are various oils on the market that will seal as well as lubricate the metal. Check Brownells and MidwayUSA. Never allow anyone to touch the metal with bare hands. Don't store it in a plastic gun case they make the metal sweat. If you live in an area with high humidity it is a losing battle.
Bluing is nothing more or less than controlled oxidation. I use Oxpho Blue cold bluing solution from Brownells. If done correctly, follow directions, I like it better than some of the hot blue techniques.