What is blue fish?

"Blue fish" can refer to a few different things, so to give you the best answer, I need a bit more context! Could you tell me:

* Where did you hear the term "blue fish"? Was it in a recipe, a restaurant menu, a conversation, or something else?

* What kind of information are you looking for? Are you curious about the fish itself, like its scientific name or where it lives? Or are you looking for its flavor profile, nutritional value, or how to cook it?

Once I have a bit more information, I can give you a more specific and helpful answer.