From the Super Mario Wiki
The Blast Box
shows its characteristic label.
The Blast Box is an explosive Crate in Super Smash Bros. Brawl. When broken, it will explode and deal 40% damage to all nearby fighters. However, the box will explode instantly when hit by a fire-based move, but this will harm the user of the attack as well if they are close enough. The box has a chemical warning sign on it signaling easily ignitable substances.
Super Smash Bros. Brawl Trophy
| Blast Box
|| Wii Super Smash Bros. Brawl
|| 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.
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