It's more akin to a block that plays itself. Which is why I bring Impeccability against the Haunted Armors in the new release. The damage type is slashing, but it isn't an attack. It is a reaction to their action, which is unblockable, because it isn't an attack. Players play attacks. You can't attack an opponent with the Barbed Platemail you're wearing, but it they attack they may feel the effect.