From Ex illis wiki
Block (BLK) refers to the likelihood that a warrior will block with his shield. Like all defences, this value is used against the attacker’s Melee Accuracy (ACC) in a Weighted Random Draw (WRD) to determine the outcome of an attack. If Block is drawn, the damage received is divided by the Shield value. Only characters with shields (or shield-like object) can Block. The bigger the shield, the higher the Block value is.
Fatigue reduces this value.
This statistic increases with each level using this formula:
Final BLK = Basic BLK X 1.029183Level (gains 33% every 10 levels)
|The Fate Screen icon shown when the WRD picks a block|