These can be found in Taverley Dungeon. Take the Blue Drag shortcut (70 agility), then run past Black Demons and Spiders.
Always carry a antifire shield, just in case, as well as a bit of food.
Attacking these is stupidly slow, due to the halberd. Also, a Dragon Halberd would be much better, but I haven't finished Regicide yet...
As for loot, black dragonhides with each kill, and of course a chance of truly amazing drops such as a Visage. Bury the bones on the spot.
black dragons blacks dragon