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.
4 possible appearances:
Location & Notes:
Located in Pillar-nest Vosh, Azj-Kahet. Summoned by Agaricus Festerbloom (located in the back of the fungal cave) during combat. Its name changes to 'Confused Swarmite' if Agaricus dies.