Blue Fruit is a way to heal the Majin. You will find these scattered throughout the world. They look like blue flowers and have sparkles floating around them. Pick one up when nearby, and then hold and to feed it to the Majin. The Majin must have lower than maximum health in order to feed it. You can have a maximum of 5 Blue Fruits at once, which is displayed by your health bar. You might get this while going through the game normally, but it's a good idea to speed it up and any time you have 5, use one when the Majin's health is down. These will respawn eventually, but it's not worth it to wait around.