illusion block you can walk through, it takes apperance of a closest block

You shoud add illusion block that changes its appearance to look the same as the closest block to it from certain direction but in reality it would be just appearance and you could just walk through it as trough water. It could be used to disguise secret places and it should propably be quite rare so that it would not be over used. It would open up all kinds of possibilites and fairly easy addition to game.