A block opposes actions in general. If people want to do anything that the block affects, they need to beat the block's strength. It doesn't stack with defense rolls, you have to pick one or the other.
Armour is only for attacks. It reduces the stress inflicted by successful attacks, essentially reducing the weapon rating.
If an accuracy 5, weapon 5 attack is thrown at a target with a 4-shift block, a defense roll of 3, and armour 2, the attack hits by a margin of 1 and inflicts 4 stress.
Normally blocks and armour are not damaged by attacks that exceed them, but magical shields are an exception. If the 4-shift block in the above example was from an evocation, it would collapse after the attack. Armour from evocations does not have that problem, but you only get half as much of it from a given spell.
Your Story 252 is probably the page you want here.