Ancient Blood can appear as certain activation, or with a chance to activate. It will always activate during the card's turn.
Ancient Blood increases ALL Healing done by targeted Creature by amount stated. This includes Heal, Healing Aura, Lifeleech and Recuperation(needs testing).
This means, if a Doctor Blood is affected by the Ancient Blood ability from Crescent Moon Pendant, he will heal all of your creatures for 3. This also means that Artifacts that heal, like Pendant of Soulmending do NOT Heal your creatures for more than 1, because it is not targeted by the Crescent Moon Pendant.
To make best use of this ability you need to pay attention to your card order! Make sure the Ancient Blood ability is activated before your Heal abilities! Check the Turn Order article to see in what order your abilities are activated. Note that Ancient Blood only increases the healing abilities of your Creatures, NOT healing done by Artifacts!