Blade Damage Booster is a gear perk that increases the damage of throwing knives and throwing axes. It makes thrown blades a reliable silent kill option.
Your throwing knives and axes hit harder, so fewer throws are needed to drop tougher targets.
Wear one of the gear pieces that carry it: Eternal Dance, Tactical Gloves. Each piece only takes one body slot, so plan around the other perks you want.
Worth it if you use thrown blades a lot for stealth. Otherwise a different hand-slot perk helps more.