It’s complicated. For passive block : your block rate isn’t a flat % of block rate. It is a flat number like 350, and the ennemy has a block rate “piercing” stat. Depending on how much of this stat he has and how much your block stat is.
For the active Block Rate : C button on peltasta OR rodolero ('cause yeah rodolero seems to have it too now) it should’t be a 100% block possible, but a boost to your block stat, so a boost to your block rate, and it buffs your defense too so that you take less damages even if your block doesn’t work ^^