"Blued" is an oxidied finish created by chemical process, similar to the way they electroplate other finishes to other products, such as gold to a coin or teflon to a pan, using electricity, acid, heat, and or other methods.
Stainless steel is just its self, except the mixture of metals determines its grade, its rustproof, and strength. Cheaper stainless steels will stain and rust, and not all as strong as others.