Well, if Dust can indeed give birth to sentient life like Humans and Faunus, I just don't see a reason why it couldn't account for the Grimm, as well. The Grimm just went a different route than humans did: Grimm commonly take the form of huge, monstrous creatures, while humans use Dust to fight back with supernatural abilities and weapons. It's like why the dinosaurs evolved into... well, dinosaurs, but humanity evolved into a highly social, intelligent, and omnivorous race to survive without relying on immense size and strength. As for the black powder, do recall that there are different kinds of Dust, and they all have different colors. "Burn" Dust is red, for example. There's also Earth, Ice, Gravity, etc. And we haven't yet seen what kind of Dust can spawn life, like it did humanity. Still, you're right, it's definitely a possibility that Grimm are from something entirely different besides Dust... I mean, we don't even know how they are "born" or whatever, yet. They're an enigma, just like the Titans from Attack on Titan (and they even dissolve upon death the same way, to boot).