Dusty Dirt, added by the New Blocks idea pack, is formed when 1 Dust is crafted with 1 Dirt. It has a unique "smokescreen" ability that is useful in traps and special effects.
When a mob or player walks over the Dusty Dirt, it will release a dusty fog making a 3x3x3 cube around the block that is exactly like the Blindness effect, except it is colored tan. This effect persists for 15 seconds after stepping out of the fog. It occupies Effect ID 24 and is called "Dust." The Dust effect also makes Creepers take 1 HP damage for every two seconds they're affected by it. However, if Zombies or Skeletons stay in it for over 3 seconds, they become immune to sunlight until they step into water (note that water also protects them; once they step out they're not sunproof anymore). However, the aforementioned mobs, if they are burning, will not seek out Dusty Dirt to protect themselves.
Effect on Skeletons
If a Skeleton stays in a dust cloud for 10 seconds or more, it becomes a Dust Skeleton. These are more powerful versions of normal Skeletons.