Diaphanous Blocks change their opacity depending on the distance to the player. By combining them with any block in a crafting table you can change their appeareance to any block that doesn’t have a tileentity. By default they don’t have a collision box and vanish when you get close to them. The inverted version you get by putting them in a crafting table on their own has a collision box and appears when you get close.
Crafting
Changing Appeareance
Inverting