I would think "This is a dark powder" but apparently Duo does not accept it. Although "noir" can indicate that something is "dark", perhaps we need "poudre foncé" for dark powder. http://www.larousse.com/en/dictionaries/french-english/noir/54393#660548
Exactly right. See neverfox's excellent post above. In a descriptive sense, you would expect to see it in a Wiki entry about a particular element or mineral, i.e., 'it is usually found as a black powder'... it is singular because it is one powder, even though it refers to all of that powder that exists...HTH
Black powder is traditionally used for muzzle loading a musket. They don't normally sell it in the stores anymore, but you can get a black powder substitute like Pyrodex, sometimes you will see a few containers of it next to where they sell bullets. Gun stores would be a better place to get black powder, I have even seen muzzleloaders there as well still called "black powder firearms.