The most important qualification to be a blood mage is to be able to do magic. Before the ban of 1970 people wanting to be blood mages could study at faculties of blood magic at academies. One of the best was the Faculty of Blood Magic of the WAMA Currently, they need to find an underground practicioner and convince him to teach them.
People have always feared blood magic because of its connection with demons and Hell. On the other hand blood mages were sought after to perform such spells like bloodline protection or lifeforce transfer.
The most common service provided by the blood mages was bloodline protection. That spell used the blood of the mages' client to create a magic barrier which can be crossed only by the client or his blood relative. It is a great way to protect valuable items or even places. Today it is the only legal blood magic spell and can done by magic-users who have a special permission from the police and passed a psychological test.
Other services of the blood mages are currently illegal, but it doesn't mean that nobody is doing them. Those are demon summoning and lifeforce transfer. Lifeforce transfer magically connects two people (primary and another person called backup or sacrifice). When the primary person dies the lifeforce of the backup/sacrifice is transfered to the primary. The backup person dies instead. If the backup is killed the connection is just severed and the primary person lives.
Most spells concetrating on the manipulation of blood are also illegal, but there is one exception. That spell is magical reanimation. Every magic-user working as a paramedic is tought it, but they can use it only when non-magical reanimation brings no effect but they believe the patient can be saved.
Dangers & Hazards
The biggest hazard connected to using blood magic, especially when a spell requires blood as a catalyst, is a risk of attracting demons to Earth. As some blood magic spells require blood sacrifice there was a risk of bleeding out, but it was minimized when the medical knowledge got better.