While Black Dragons can be profitable due to their dragon bones (good for Prayer XP),they are not ideal for efficient Slayer training. These creatures have high Hitpoints and require you to bring either an Anti-Dragonfire Shield or an Antifire Potion,adding extra prep time to each task. For maximum efficiency,you'll need to use a Dragon Hunter Lance,which is expensive and generally reserved just for Slayer tasks. Since Duradel assigns a relatively low number of Black Dragons to RuneScape gold kill per task (around 10-20),it's better to block them and focus on monsters that can be killed in bulk for greater XP gains.
Greater Demons & Black Demons

Both Greater and Black Demons are high-weight tasks with disappointing loot tables. Duradel often assigns large quantities of these demons (up to 200),which means a lot of time spent fighting slow,single-target monsters with very little in return. On top of this,finding a world without competition can be a hassle,and the spawn rates aren't always ideal. Ironman players may consider keeping these tasks since they drop items needed for Slayer Helm upgrades or Emberlight,but for most players,these demons aren't worth the time and effort.
Spiritual Creatures

Spiritual Creatures found in the God Wars Dungeon (GWD) are notoriously annoying to deal with. These monsters have low Slayer task weights but are widely considered some of the most tedious to kill in the game. They require a significant amount of preparation,as you'll need specific God-aligned gear to avoid constant aggression from other GWD monsters. Their loot table is also poor,so the task isn't worth the hassle unless you're specifically after a pair of Dragon Boots. For me,these creatures were the first I blocked,and I've never regretted that decision.
teel Dragons

Steel Dragons,like Black Dragons,are high-defence,high-Hitpoint monsters that can be tedious to kill. Their primary attacks include a strong dragonfire breath and powerful melee attacks,so you'll need to bring protection prayers like Protect from Melee and Piety to maximize your efficiency. Although the introduction of Project Rebalance has made Steel Dragons weak to Earth Spells,they remain a slow kill,and their loot table isn't very rewarding. Steel Dragons often come in small task sizes,and even with Earth Spells,these tasks can take far longer than preferred. Given their high difficulty and low reward,Steel Dragons are a prime candidate for buy RS gold blocking.