What makes a perfect block perfect?
These quality levels are based on timing, and they are very unforgiving. Your action must occur very close to the moment the enemy would hit you. Note that timing applies to parry and dodge as well as block. There are actually three levels: normal, great, and perfect.
Unlike a normal block, a perfect block causes your shield to lose no durability. Getting a perfect as the last in a series also stuns the enemy slightly longer. When this happens, you'll see "Perfect Block Break". Additionally, they are sometimes requirements for a bonus (e.g. Perform X perfect blocks).