It seems that monsters have a hidden accuracy stat that in some way influences dodge, block and parry rates. This could explain how block rates seem to stay roughly the same as you level up and equip better shields even though GLA/PLD shield "Block Rate" stats increase from 35/10 to 181/181 & 295/92.

If Bulwark is actually modifying the "Block Rate" stat this may explain how a +60% increase seems to lead to a 100% block rate when used in overleveled content. Holy Shield's "Block Rate" under Bulwark would be 289.2 and Onion Shield's would be 472.