I can't seem to find any consistency for EntityDamageByEntityEvent using a shield.
On hard difficulty:

Zombie without blocking:
Damage: 2.5
DamageModifier.BLOCKING: -0.0

Zombie while blocking and facing TOWARDS it:
Damage: ~0.825
DamageModifier.BLOCKING: ~0.087 <-- It's actually increasing the damage!!??!?

Zombie while blocking and facing AWAY from it:
Damage: 2.5
DamageModifier.BLOCKING: 0.75 < Lowers damage while facing away? Huh?

PigZombie without blocking:
Damage: 5.0
DamageModifier.BLOCKING: -0.0

PigZombie while blocking and facing TOWARDS it:
Damage: ~1.65
DamageModifier.BLOCKING: ~0.35 < This one actually lowers it (like I'd expect it to)??

PigZombie while blocking and facing AWAY:
Damage:5.0
DamageModifier.BLOCKING: -2.0 <-Reduces damage.. while not even facing it?

Various other mobs provide varying results too, there doesn't seem to be any pattern.
How are we meant to test if the attack was blocked by a shield if the blocked damage can even be POSITIVE?