From the Super Mario Wiki
A Bomb Block is a type of block that also serves as a platform in Wario Land: Shake It!, and is seen in many levels. They have a certain number on them (usually less than four), and when Wario walks on them, the number will count down and cause the block to explode. Bomb Blocks are also very sensitive, as they can explode when Wario shakes the ground. Bomb Blocks usually appear in desert levels and can sometimes hold obstacles back, such as huge boulders. These blocks can also be used to defeat enemies as the blast from the explosion will harm them.