Non cumulative, meaning you can't gain another additional bonus from bloodlust when you kill another enemies, say +2 damage and 3 armor then it can't be +4 damage and 6 armor if you kill 2 enemies in a row. To make it short, it is non stackable for another Bloodlust bonus but it can stack with other skills like Duel, Heavy hand, and even Battle Prayer or Arbenos Might. Bloodlust simply overwrites another Bloodlust bonus if you kill another enemy along the duration of it.