When an attack is Blocked, it does not necessarily mean that the target takes no damage. A Block attack-result inflicts the same damage as a normal hit, but reduced by an amount "X" that's based on the Block value and Strength of the target, as follows:
This enchant is often confused with Enchant Shield - Shield Block - which adds + 15 block rating they are not the same thing. That enchant increases the chance an attack will be blocked by 15 (approximately 1.9% at level 70) while this enchant reduces the damage inflicted by a blocked attack by 18.
Block Rating is more valuable than Block Value from a damage mitigation point of view, however bear in mind that Block Value is used to determine the amount of threat generated from a warrior's Shield Slam ability. As a tank's ability to mitigate damage increases, the amount of rage received from combat decreases. Thus, in situations where a tank is receiving less damage, but still must maintain threat, Block Value becomes essential. An example of this would be a level 70 warrior tanking Mana Tombs or Sethekk Halls. It is common for a warrior tank to have a "Raid Tank Set" and a "5-man Tank Set," where the first focuses on damage mitigation, and the second focuses on Block Value.