Another flavor of carbon steel, but with a bit of pre-treatment which gets the "non-stick" action started, and supposedly promotes a little extra and earlier crust and/or fond. Blue steel pans are usually a fairly light gauge -- and you often see a "Lyonnaise" style curved wall in blue steel -- as opposed to the "ordinary" straight flare you ordinarily see in ordinary carbon. [The Lyon shape is supposedly an aid to toss turning, but you can't prove it by me. If you can toss you can toss, and if you can't you'll get (more) stuff all over the stove.]
Otherwise, not all that different from other carbon pans; same strengths, same weaknesses, same care, same caveats, same-same.
Hope this helps,