Places a Blessing on the friendly target, reducing damage taken from all sources by 3% for 10 minutes and increasing stamina and strength by 10%. In addition, when the target blocks, parries, or dodges a melee attack the target will gain 2% of maximum displayed mana. Players may only have one Blessing on them per Paladin at any one time.
Damage taken reduced by up to 3%, stamina and strength increased by 10%, and blocked, parried, and dodged melee attacks cause a gain 2% of maximum displayed mana.
Blessing of Sanctuary is a talent in the Protection tree, you will need to spend 21 points in Protection to max it. This blessing is primarily intended for tanking paladins, as it requires a significant investment into the protection tree. See [Greater Blessing of Sanctuary] for the 30 minute version of this buff.
This buff overwrites the warrior [Vigilance]. Similarly, Vigilance overwrites Sanctuary. No longer so as of 3.2.0. In patch notes this fact is mentioned in bug fixes section.
Tips and Tactics
This spell is most useful when faced with a number of quick hitting mobs of lower levels as it increases the amount of times you are avoiding attacks, and therefore increases mana regeneration. This talent is best used in conjunction with [Holy Shield] in order to increase the amount of mana returned.
This blessing is a key part of Paladin AoE grinding. However, when dealing mainly with single target, a protection paladin may be better off using [Blessing of Wisdom] to reduce downtime.
Normally, only Protection Paladins have this blessing. DPS classes find the 3% damage reduction pretty uselsess and other tank classes dont use mana (See Moonkins as Tanks
Patch changes
Patch 3.2.2 (2009-09-22): This blessing now grants 10% strength in addition to its current effects. Also, the strength and stamina bonuses from this blessing will no longer be lost when Blessing of Kings is removed.
Patch 3.2.0 (2009-08-04): This blessing now also increases stamina by 10%. This effect is not cumulative with [Blessing of Kings].
"...we want to make sure Blessing of Sanctuary is always the tanking blessing of choice" (source)
Patch 3.1.0 (2009-04-14): Now only grants mana on dodge/parry/block. In addition, will only grant mana if that is the active power type of the friendly target (Bears and Cats won't gain mana).
Old, gain 10 rage , 20 runic power, or 2% of the maximum mana
Patch 3.0.2 (2008-10-14): Reduced to 1 rank, now reduces all damage taken by 3% and generates rage, mana or runic power when a parry/dodge/block occurs.
Old, 5 ranks. Last rank text was "Places a Blessing on the friendly target, reducing damage dealt from all sources by up to 80 for 10 min. In addition, when the target blocks a melee attack the attacker will take 46 Holy damage."
[Blessing of Sacrifice] and 
[Blessing of Salvation].
[Greater Blessing of Sanctuary] for the 30 minute version of this buff.
[Vigilance]. Similarly, Vigilance overwrites Sanctuary. No longer so as of 3.2.0. In patch notes this fact is mentioned in bug fixes section.
[Holy Shield] in order to increase the amount of mana returned.
