The Dark Cloud is a block added by Natura. Dark Clouds can be found spawned naturally at around Y=200 and in The End, making them very difficult to obtain. Entities will not be able to fall through the bottom of the block, or the bottom block of a structure. Entities that have fallen into a Dark Cloud will not take any fall damage. Dark Clouds break fastest with swords, but can be harvested with anything.
Although the Dark Cloud's tooltip states that it "causes lightning strikes," the Dark Cloud has no such ability.