"What if bless armour stuck to adding protection, but that protection was based on where the damage was coming from rather than a static amount?"
If an undead hits a body part which has an item with blessed armour, there is a certain chance of a holy critback...? Only versus undeads, nothing else.
Oh, and if an undead (lich or ghoul) wears armour that's blessed, there is a small chance each round of them being holy critted.
That would be fun.