Try lightly rubbing with oil on a patch or cloth. Or just use a clean toothbrush and oil. Lightly. Then leave it oiled overnight and do it again. Let us know how it turns out.
The blue on a gun is just a form of rust in the first place, and some continue to rust a little faster than others if not oiled, greased or waxed. They all rust eventually, even the stainess guns, if they're exposed to water or fingerprints.
I think the inexpensive Remington shotguns, based on my Express and Wal-Mart 1100 synthetic, are put in the box before the blue (or preservative) has dried because it was all over everything.
John