Gunpowder Block is a custom item made especially for Blackblock.
Just like a TNT block, the Gunpowder block will cause an explosion when lit by a flame arrow or flint & steel.
Be warned however, the explosion is instantaneous and will destroy blocks. (TNT doesn't destroy blocks anymore, it just drops them)
Just like sand & gravel, the Gunpowder block has gravity and will fall to the ground.
(Because of the way it's currently implemented, you won't actually see the block falling. It'll just appear at the bottom)