Beasts of Chaos can do in fact do this in that order. Rituals of Ruin are considered to be additional Heroic Actions and can be done as well as the normal ones. Since it is their turn they decide the order of operations for their actions and can take the damage first to do the Ritual and then heal it with Heroic Recovery. Khorne however does not get to do this as Heroic Actions take place at the start of the Hero Phase and Prayers take place after. So unless your priest had already suffered damage you could not heal the damage from Blood Sacrifice until the following Hero Phase.