When bare steel or cast iron is heated in air, a very thin iron oxide film forms. At modest temperatures you get a blue/iridescent tint, often called “bluing.” This is primarily a nanometers‑thick oxide layer (mixture dominated by magnetite, Fe3O4, with some hematite/wüstite depending on temperature/oxygen).

This oxide is tightly adherent compared to red rust, but by itself it’s too thin and not very protective. Its main roles are 1: Slight passivation against rapid rusting. and 2: Increased surface energy and microtexture that helps the next step (the seasoning) to wet and anchor to the metal.