Bluing is a very thin coating on the steel made by converting the top layer of steel to a blue-black color.

The color depends on the steel composition and how well polished the surface is before it is blued.

Bluing uses salts in a very high temperature solution (~380 F boiling point there is so much salt in the water).

Blued steel needs to be protected with another finish or oil to prevent rust.

Without some protection even fingerprints can lead to rust.