Blessing of Kings is a core paladin ability learned at level 30. It buffs strength, agility, and intellect by a small amount.
Blessing of Kings applies its bonus to stats after all other buffs and auras on the target are added, making the cumulative effect potentially greater than any other class buff.
Blessing of Kings is not as effective as it was before TBC; gear then mainly boosted primary attributes: strength, agility, intellect, spirit, and stamina. Itemization since TBC emphasizes secondary attributes such as attack power, critical strike rating, and spell damage, which are not directly improved by Blessing of Kings.
In PvP, it is usually the best blessing for any class, as having high stamina is vital in surviving enemy players' attacks.
Friendly boss characters (like capital faction leaders and the 2 generals in Alterac Valley, Vanndar Stormpike and Drek'thar) are immune to Blessing of Kings, perhaps because the added bonus would be too strong on those NPCs.
Patch 5.3.0 (21-May-2013): Blessing of Kings had its mana cost reduced to 5%, down from 22.3%.
Patch 4.0.6 (8-Feb-2011): The mana costs of Blessing of Might and Blessing of Kings have been increased by approximately 217%, making them roughly equal to the cost of Mark of the Wild.
/Patch 4.0.1 (12-Oct-2010): Is now trainable at level 22, provides a 5% bonus to Strength, Agility, Intellect, and Stamina, and provides an amount of spell resistance dependent on level (65 at level 80). Lasts 1 hour, and affects all members of the target's party or raid. No longer stacks with Mark of the Wild.
Patch 3.3.0 (08-Dec-2009): It is no longer possible to have multiple buff icons from this spell.