The Blast Box (also called a Danger Box)- known in Japan as Explosive Box 爆薬箱, Bakuyaku Bako, is a heavy item which has the appearance of a crate with a fire hazard label on its sides. Blast Boxes are quite dangerous items, when destroyed, the box will explode with typical fire properties and great vertical knockback, often KOing anyone too close. It usually takes multiple hits to destroy it (having 30 health), but it detonates instantly when attacked with a fire-based attack or very strong attacks. This item can also be picked up and thrown a very short distance (usually just in front of the character that threw it). Although it is a heavy item, it can appear out of carriers such as Barrels and Capsules. When down smash thrown at point blank range, the Blast Box will KO almost as quickly as the explosion.
A Blast Box does have a slight amount of tactical strength when playing a match with items. It can be remotely detonated by using ranged attacks (especially fire-based ones) and thus, with a well placed projectile and good timing, it can be detonated while an opponent is near.
 Description from the Super Smash Bros. Brawl instruction booklet
Explodes from a strong attack or fire.
 Trophy Description
An explosive-filled box that will detonate when the box is broken. The box can be smashed with normal attacks and will also catch fire and blow up if it's exposed to any flame. Unlike ordinary crates, blast boxes do not change their appearance from stage to stage. There's one fundamental rule when dealing with these items: be out of range when they blow.