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Block rating is number typically appearing on items that improves your chance to block an attack. Some abilities, enchantments, items and spells can contribute bonus block rating also (the bonus spell usually has the name "Improved Blocking" or "Increased Block X").
Block rating is not to be confused with "block value" which — prior to the Cataclysm expansion — increased the amount blocked (damage mitigation) each time a block occured, rather than affect the chance for a block to occur.
Change in Cataclysm
Blocked attacks will simply hit for 30% less damage. Block rating will improve your chance to block, though overall block chances will be lower than they were in Wrath of the Lich King.
Block chance used to also be increased through defense rating, of which 122.4 rating is needed for 24.88 defense to give 1% block chance, but defense rating was removed with the Cataclysm expansion.