It is grindy-type weapon. You just need to do the killing blow to enemy with it. Even minions will do the job while you have it in hands.So how does bloodthirsty blade actually work? I have been using it for a while now (5-6 in game days) and it still hasn't leveled up. Tried to kill EoC and KS with it and nothing happened.
You are getting bloodthirsty points after killing the enemy. When you are getting around 100-150, it gets +1 damage.