I came across a in the white mid length barrel & know I could finish it by bluing among other finishes but I actually really like the finish, it looks like a stainless barrel to me (by the pictures anyway)
How easy will it rust under occasional use?
I personally think you will have problems with it. Especially if you are in a damp or humid climate. Bluing is a form of oxidation that brings out the black oxide in steel and with a light coating of oil is pretty good protection from rusting while "rust" is oxidation that brings out the red oxides in steel that is very harmful.