The Blue Moon is a Flail that can be found in Dungeons. When used, it will emit light around it. The Blue Moon is the second weakest flail in the game, being stronger than the Ball O' Hurt but weaker than the Sunfury. When the Blue Moon is thrown far from the player or hits a block, it will quickly retract, throwing the weapon behind a line of enemies will force it to retract through them all, dealing damage to all of them. There is another way to use the "retract method" as mentioned above. If you turn around when it's coming back to you, you will swing it the other way; you can do this an infinite number of times, as long as you time it correctly. It looks like you are spinning it around and around. The Blue Moon is especially effective in dealing with Goblins, groups of enemies and enemies with multiple sections, such as Eater of Worlds.
When throwing the Blue Moon, if you just click the mouse to throw it, it will only be thrown a short distance away from you, but if you click and hold, it will go further.
If clicked and held, the Blue Moon will stay out and will flail around the player.
Even though it is better than the Ball O' Hurt, it will sell for the same amount to all vendors.
Sometimes it can be found in chests nearby the Dungeon, just like the Dungeon Guardian can spawn when nearby, so keep an eye on how close to the Dungeon you mine.