Blast Mask

The Blast Mask is given to you 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 you need a bomb quickly, but have none in your inventory.

The downside to using the mask is that it does damage to you every time you use it. However, if you use your shield while using the mask then you will take no damage.

Obtaining The Mask
The Blast Mask is one of many items you receive for completing entries in the Bombers' Notebook. You will receive it after you complete the entry for the Old Lady. Fortunately, her entry is the shortest in the entire notebook and you can complete it immediately after you have received the Deku Mask.

Visit North Clock Town on the night of the first day. At midnight, the Old Lady will enter town. At exactly 12:15, the thief, Sakon, will steal her bag full of Big Bomb Bags. Attack Sakon with your sword (anything else used against him will blow up the bombs, killing Sakon and preventing you from getting the mask) to make him drop the bag. He will then prance around before leaving town.

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