Blast Mask

The Blast Mask is given to Link by the Old Lady from the Bomb Shop in Majora's Mask. While wearing the mask, the sword icon on the B button will replaced with the word "Explode". If B is pressed, an explosion will occur from the mask, much in the same way that a Bomb explodes. It can be especially useful if a bomb is needed quickly, but have none in the inventory.

The downside to using the mask is that it does damage to Link every time he uses it. However, if Link uses his shield while using the mask, then he won't take any damage.

The mask is transparent after explosion for a set amount of seconds. The mask cannot be used during this time, neither can the sword until the mask is taken off

According to a Gossip Stone, the mask is filled with gunpowder, enabling its exploding abilities

Obtaining The Mask
The Blast Mask is one of many items that can be received from completing entries in the Bombers' Notebook. Link will receive it after he completes the entry for the Old Lady. Fortunately, her entry is the shortest in the entire notebook and can be completed immediately after receiving the Deku Mask.

On the first day at midnight, the Old Lady will enter town through the north gate. At exactly 12:00am, the thief, Sakon, will steal her bag full of Big Bomb Bags. Link must attack him with his sword (and nothing else, otherwise the bag will explode), causing Sakon to drop the bombs and preventing him from stealing the mask. He will then prance around before leaving town.

After Sakon has left the scene, the Old Lady from the Bomb Shop will reward Link with the Blast Mask.