The Blast Box (爆薬箱, Explosive Box), is a heavy item that appears in Super Smash Bros. Brawl and Super Smash Bros. 4, which has the appearance of a crate with a fire hazard label on its sides. Blast Boxes are dangerous environmental hazards due to their explosive nature; when destroyed, they explode with a fiery explosion similar to those created by Bob-ombs, but larger. The explosion causes great vertical knockback, usually KOing anyone at 30% or above. Blast Boxes usually take multiple hits to destroy (having 30 HP in Brawl and 20 HP in Smash 4), but they detonate instantly when attacked with a fire-based attack or single, strong hits. This item can also be picked up and thrown a very short distance (usually just in front of the character that threw it).
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. Some attacks with invincibility or super armor can also be used to detonate a Blast Box at close range without taking knockback.
Humorously, despite its size and classification as a heavy item, it can appear out of carriers such as Barrels and even Capsules. Additionally, it does very high knockback if down smash-thrown at point blank range, being able to KO almost as quickly as its explosion.
For some reason, if Ness in Smash 4 detonates a Blast Box with his side smash. he will be immune to the explosion.
Description from the Super Smash Bros. Brawl instruction booklet
Explodes from a strong attack or fire.
In-game description from Super Smash Bros. for 3DS
Watch out—if you hit this with a fire attack, it'll explode!
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.
A dangerous box that has been packed full of gunpowder. The player who sets off the box, either by hitting it or throwing it, will get points for any KOs. Setting off the box will result in a massive explosion, and you've only yourself to blame if you blow yourself up. It says DANGER right on the front.
A dangerous box packed full of exceedingly flammable gunpowder. Fire and strong attacks will set it off, and the explosion will be massive, so keep your distance! By the way, any points for the KOs caused by the blast go to the person who last threw the box. If no one threw the box, nobody gets the points!
If at least 4 Blast Boxes explode at the same time, then the game will stop all sounds (except the explosions themselves) and voices being played. This happens in Brawl and For 3DS.