If you are fond of some challenging class to play, then you probably will go in for the Blood Mage, since it requires not only the basics, but also the most important one: the player's skills. They are characteristic of their blood and also their enemies'.
Blood Union
A Blood Mage can form a parasitic union between his own blood and others called a Blood Union. Once this Blood Union is created the Blood Mage can use the blood of his enemies to benefit himself and his allies. Blood Unions can range in level from 0 to 5.
Weakness
A weakness is a player created vulnerability in your enemy that remains for a short period of time. Some weaknesses can be exploited by using certain skills. When one exploits a weakness the attack used becomes more powerful and may add more than additional damage but may debuff the enemy or buff you or your party.
Buffs
Blood Mages have many buffs including damage shields, increases to intelligence, vitality, wisdom armor class and hit points, symbioses and more.
Reagents
The Blood Mage can take the organs from his enemies using the Grim Harvest spell. Blood Mages can also siphon blood from their own veins. These reagents can be used in creating symbiotes and to cast powerful spells such as Life Husk.
Direct Damage
The Blood Mage is the most arcane of the healer classes. She has no special melee attacks and almost all of her damage is spell damage, primarily direct damage, DoTs and lifetaps.
Healing
The Blood Mage is able to use his own blood and the blood of his enemy to heal himself and his allies. Blood Mages can form a bond with an ally or allies to share or split incoming damage.  He can also heal his allies by stealing the health of his enemy and directly transferring that health to his ally. The Blood Mage has one instant heal, called Ritual of Gelenia, witch consumes Blood Union points. Blood Mages also have traditional heals such as single target heals over time, and group heals.
The Blood mage can choose from a variety of ways to heal.
Pacts
Blood Mages get two main pacts: Scarring Pact and Blood Pact.  Scarring Pact allows the Blood mage to share damage with one player and Blood Pact allows the Blood mage to spread the damage across the entire group.  Pacts are situational ways to heal and not very energy (mana) efficient.  They allow the Blood Mage to moderate damage spikes.  Often pacts are used to heal when a light fighter is tanking, the tank is having problems holding aggro or a PSI is taking damage with Thought Pulse.  One must be aware to always be ready to cancel pacts to avoid unnecessarily killing a group mate.
Entwining Vein and Blood Tribute
One of the main ways Blood Mages normally heal. Entwining Vein steals health from the Blood Mages enemy and returns 130% or more of the damage to his defensive target. Blood tribute returns a smaller portion of damage but works like Entwining Vein. Both are mana efficient ways to heal.
Heals over Time (HoTs)
Blood Mages have two HoTs, one very powerful--Flesh Mender's Ritual.  Both are very energy efficient ways to heal.