Bliss is a gear perk that boosts your weapon damage while you are poisoned. It turns a status effect that normally hurts you into a reason to fight harder.
While the poison effect is on you, every weapon you use deals more damage.
Wear one of the gear pieces that carry it: Prophet's Shades. Each piece only takes one body slot, so plan around the other perks you want.
A niche perk. It shines in poison-heavy fights, but it does nothing the rest of the time.
Combine it with poison weapons and the Fantasma Supremo, so poison is often in the air.