I have (had) both a blued and a park'd 590. (I say had because when they were recently refinished after a hosue fire, the gunsmith misunderstood my instructions and blued them both instead of park'ing one of them.)
Anyway, the blued gun always rusted at the least little hint of moisture. I swear that was the thinnest bluing I have EVER seen on a gun. Eventually it kinda looked like it had spots.
That said, my recommendation is for you to go buy a $50 Duracoat EZ-Finishing kit and Duracoat the gun yourself. The kit containes everything you need to do 2 long guns including the Duracoat, reducer, degreaser spray, airbrush setup and an instructional DVD. The only bad thing is that I haven't had good luck getting a DVD that I can actually watch from start to finish as they have some sort of formatting problem. Never fear, they're working on a good old VHS instructional film as we speak.
Duracoat. That's what I would, and am going to, do. You can even choose what color you desire.
DuracoatEric