Most of my handguns and long guns are blued. With the exception of one, rust has not been a problem. My 'bird' gun, an old Browning Citori, has been hunted hard (today in the rain, for example) for at least two decades killing thousands of quail and doves. The bluing is totally worn away from the bottom of the receiver from carrying. Not a spot of rust anywhere on it. My main carry gun is blued. It never rusts. A little care is all it takes on most correctly blued guns.
Now, my problem gun is a different story. It is a Sako .223 and its barrel will rust in a matter of days if not reoiled. I read somewhere that the bluing process contains a 'stop bath' that is supposed to stop the bluing (really a kind of rusting), and if the stop bath is not used correctly, the bluing/rusting never truly stops. This may be the problem with some of our 'problem guns.' I had a Ruger 77 a few years ago that had this same problem. I tried every rust preventative known to mankind on my Sako. Nothing worked until one day I tried some Johnson's Paste Wax. No rust for years now, even though I hunt it pretty hard most winters. I put a new coat of wax on each year. No more oil for the exterior of that gun.