When you battle Blocky, he can use two different attacks. He can jump and then slam into the ground, which will cause two rocks to fly out of the impact. Blocky can also charge into a wall, also causing a rock to fly out of the impact. These debris rocks can be inhaled and then spat back out to attack.
Blocky was originally an enemy from Adventures of Lolo, an NES game which was also made by HAL Laboratory. In this game, Blocky would run toward the player with the intention to block their way, and to make them stuck, forcing the player to restart the level by pressing select and dying.