Blood Weapon is a Dark Knight ultimate technique, which allows them to steal life energy through their weapon and add it to their own power. The greater the wound one inflicts on their enemy, the more energy they are able to steal. Unfortunately, the undead have no life energy within them, so this ability is ineffective on them.
When active, all standard melee attacks will cause an Additional Effect that converts the damage inflicted into a "Drain" effect, healing the Dark Knight by 100% of the amount of damage inflicted. No additional damage is inflicted on the target, similar to Drain Samba.
This ability has no effect on WS.
While Blood Weapon does not affect Weapon Skills, Additional effects DO proc on WSs.