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.
Found in several locations in the zone, including the the webbed tunnel that leads east from Ruptured Lake towards Siegehold (north-west of zone), and the wilds wilds north and south of Rak-Zakaz and Crawling Chasm in the north-east.
Found just east of the large Royal Preservatorium building.
Level scaling: Most NPCs will scale with the Hunter's level, within the constraints of their level range. Hunters can tame regular NPCs up to 2 levels higher than them, but can only tame elite NPCs of the hunter's level or below.