(last updated: 12/28/17)
Each time you attempt to block an attack, you will incur a "block penalty." This block penalty represents the notion that fighters can't keep blocking forever; sooner or later, their defenses will be worn down.
Block penalty starts at 0%. Block penalty increases with each attempted block, regardless of whether the attempts are successful or not. Block penalty can be decreased by dodging, and by completing an action.
Stronger attacks deal higher block penalty than weaker attacks. An attack with the Crush effect deals twice the block penalty of an equivalent attack without Crush.
Note that while you do still take block penalty even if you fail to block the attack, the penalty you build up is less than half of what you would have gotten if you had successfully blocked the attack.