It's bluing over unsmoothed and polished metal. That finish does hold oil better than a glossy blue. Neither finish will withstand as much abuse as the Phosphate finishes we generically call "Parkerizing", but the Express finish is not that far behind, especially in salty environments.
I've used Remoil, CLP, and now SLIP 2000 mostly on the outside of my shotguns and they work well. Some guides I know swear by using a coat of auto wax on the exterior on any shotgun renders it impervious to moisture, tho I note some of these are pretty beat up looking.None seem rusty.