Vulturyol is the best known dragon in Blackreach. It can be summoned by using the Unrelenting Force. Shout at the “giant lamp thing” that is above your head and he doesn’t really have any interesting loot. If you hide from him or don’t engage him in combat he will kill everything else in Blackreach for you, but bare in mind he can’t be summoned more than once in a play-through.
Here is a report from reddit user ironwolf56 : I found Vulthuryrol completely by accident. It was only a couple weeks after Skyrim had come out and he wasn’t well known at that point. I was fighting a bunch of the Falmer in the center of Blackreach, Unrelenting Shouted one of them off the upper platform and hit that globe with the shout too. Suddenly there’s a dragon around!