It's simply just a square block that has an associated "health" with it. If the block has full "health," you draw it as a full green block. When you shoot it, it looses "health" and, depending on how much "health" it has, you just draw different versions of the block.

You could easily create a set of images, the first one being the full block with full health, the next image being the block missing a few pieces after being shot once, the image after that missing a few more pieces after being shot again, etc. Then, for each time you shoot the block, you just move down the list of images that show a slowly degrading, crumbling block. Once the block looses all its health and is destroyed, you just remove it from the game.