While the Dragon Bones are good for RuneScape gold Prayer XP,the time spent killing Black Dragons doesn't justify the rewards. Duradel may only assign you a small number of Black Dragons (10-20),which isn't ideal for maximizing XP. What you're really after are tasks with higher kill counts that can be done quickly and with efficient methods.
Greater Demons and Black Demons

Both Greater Demons and Black Demons are single-target,high-weight tasks that can be a nightmare to grind through. Duradel often assigns large quantities of these monsters (130-200),which means you'll spend a lot of time killing them with little XP return. These demons don't drop anything particularly valuable,making the task even more frustrating.

Additionally,Greater Demons and Black Demons are often found in crowded locations,where you might have to compete with other players for kills. The lack of good loot and the lengthy kill times make these monsters inefficient for cheap OSRS GP training Slayer.