As everyone has learned, the world is not purely black or white. They are gray areas in which the concept of good and evil is blurred. This is also true for the elements of Moon and Blood.
Occasionally, elementals from either clan, for some reason, decide to switch sides. As such, there are elementals that can control Blood but fight for the Heaven element, and vice versa.
These beings are called the Risen and the Fallen. (Cliche, I know)
The Risen are blood elementals that have tasted good and enjoyed it.
The Fallen, alternately, are moon elementals that have tasted evil and crave more.
There is a process for both, that each elemental who wants to switch must spend a lifetime as a human, and decide whether or not they enjoyed committing evil or practicing good. At the end, the elemental faces the leaders of both the blood and the moon elements and tells them their decision, whether to stay with their element or switch. If they decide to switch, the leader of their original element signs off their soul to the opposite element, and the opposite element will receive them and give them a piece of their race.
The Moon will give them a drop of crystallized moonlight, and Blood will give them a drop of crystallized blood originating from the leader of the Clan of Blood.