While the Dragon Bones are good for 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 RuneScape gold 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 training Slayer.
Spiritual Creatures

Located in the God Wars Dungeon,Spiritual Creatures are infamous for being some of the most annoying monsters in OSRS. Not only do they require special God-Armor to avoid constant attacks from other creatures in the dungeon,but their drop table is equally unremarkable.

These creatures can be incredibly tedious to deal with,especially if you're trying to grind through multiple tasks. Unless you're specifically aiming for Dragon Boots,blocking Spiritual Creatures is a wise decision to save yourself the headache and inefficiency.
Steel Dragons

Steel Dragons are another creature that should be avoided. They are notoriously difficult to kill due to their high defense,strong melee attacks,and dangerous dragonfire breath. To deal with them,you'll need to bring along anti-dragonfire gear,and you'll often find yourself praying Protect from Melee and using Piety for most of the kill.

While they drop Dragon Bones,which are useful for Prayer XP,the time spent on Steel Dragons isn't worth the reward. These dragons are often assigned in small quantities,but the long and dangerous kills can quickly become frustrating. Since the loot isn't great and the task is time-consuming,Steel Dragons are another candidate for buy OSRS gold your Block List.