herald of blood skill gem description

"While active, killing an enemy with Blood Loss will cause a bloody explosion that deals Physical Attack damage to surrounding enemies based off the life of the exploded enemy, destroying their Corpse if the enemy was Normal or Magic. The explosion also has a chance to Aggravate Bleeding."

this skill does damage based off the blood loss the enemy has sustained, not the life of the enemy.

EDIT: it also appears to have a base physical damage as well.