You must learn a special skill to tame Blood Beasts. It is taught by a Blood-Soaked Tome of Dark Whispers, a guaranteed drop from Zul in the Uldir raid on Normal Mode or above (i.e. not LFR). Once learned, this skill is account-wide.
2 possible appearances:
Location & Notes:
Located in Maddening Deep, Azj-Kahet. You must have the Unseeming Shift debuff in order to see this creature. This debuff is bestowed during the Melody of Madness quest chain, but you can also gain stacks of it in the Maddening Deeps by clicking on the Black Blood Extractors or standing in the ashen-red pools.