Blocks that have been placed by other players on the server show up and are usable. (eg they can stand on them and move. When another player on the server is watching them place the bricks, the ground under turns dark and the block is invisible. Attempting to stand on the block if you are not the placer of the block causes you to vibrate. When the placer of the block logs out of the server and back in, they also loose the ability to see the block