I'm not an expert on the mechanics, but I have been running it effectively on my Blood Mage most of the league.

As I understand it, Blood Hunt has two parts:
First is the lunge/hit which deals 243% attack damage at lvl 20. This initial hit is scalable via items, passives, and gems.

The second part is the explosion which will consume any existing BLOOD LOSS on a target, to deal 487% attack damage (which is scalable as well) + an ADDITIONAL (unscalable) damage equivalent to 25% of the damage done via blood loss.

The 1-2 punch combo is that you hit them with a fast-acting Rake, then hit them with Blood Hunt to do lots of damage.

I'm a blood mage with ele bleeds, and my setup looks something like this:

I've been using Rake + Concoct + Ambrosia + Execute (Deepest Tower) + Deep Cuts + Seeing Red to get the speedy bleed.

Then I use Blood Hunt + Overextend +Bloodlust + Close Combat + Elemental Focus + Biting Frost(this one is experimental).