Shulkers are not disguised really as a block

They can't be retrackted with sticky pistons, and the whole block turns red when you try to hurt it, not just the shulker inside. When they are closed, you should just try to mine them as a real block (particles, sound), but you can't, like bedrock. The block would break, if you kill the shulker inside. The shulkers are also a bit smaller than a normal block.