knife blued itself?

It's hard to tell in the picture but I was using my knife to cut deer meet and then I washed it off and dried it. I forgot to oil It so when I checked it today to see how bad it rusted it formed a blue tent across it and did not rust at all. How could this of happened.

Generally folks call that "patina"...... its a form of oxidation that is a fact of life with any carbon steel. The reason it happened is that there wasn't enough moisture to create rust, but the slightly acid content from the deer meat/blood left enough residue to cause the patina. Had it been oiled or waxed, oxygen could not have gotten to it, and no patina would have formed.

Bluing like on a firearm is just a controlled rust.

patina is a beautiful thing........

Gotta love carbon steel characters.