You must learn the skill to tame Blood Beasts, which 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.
Location & Notes:
Located in The Underrot (Dungeon). Infested with many ticks that are lost upon taming.