Blood magic probably exists in areas other than Dexia since it could appeal to the sorts of people who like the idea of gaining power from hurting people. Dexia is an area that helped encourage the use of blood magic due to the fact that there was no way to tap ley lines / use magical stores in the ground. Since you can't use the ground to power spells, spells in Dexia have to either be lower powered or more efficient.
How does it work? To get around the low magic problem, blood magic sets up a set of fake leys that are as balanced as best as they can be. Emotions are siphoned into these ley lines and are stored in a chancy sort of system that balances out all the opposing forces. Should the leys fall out of balance, there's a high risk of some sort of backlash. Common backlashes include - a sudden output of heat, a sudden output of wind, a sudden explosive force, cracking of nearby stones, and fires. Weaker leys can fall apart with milder versions of the above. The riskiest parts of using blood magic is, in order of least dangerous to most, when the spell is set up, when the spell is taken down, and when there's a sudden power surge into the spell.
Early Blood Magic The earliest blood magic spells were actually used as sort of house blessings. Someone would set up a ley lines and the passive absorbing spells in a meeting room or a church, and then the spell would be set to, for example, protect the house from being burned, or to help people heal faster, or to help keep the garden warm in case of an early frost.
These spells were intended to last for centuries and were mostly very low powered to the point that most people don't even notice the spell.
Later Blood Magic Blood magic's bad reputation didn't come till later. Later blood mages began to realize that the fastest way to gain energy was to hurt people. While blood magic will store energy from any sort of emotions, it's best to have a fairly constant emotional source. Pain and anger is a lot easier to keep fairly constant compared to something like happiness.
Since they were hurting people, blood mages also discovered that blood itself could be used to target a spell. Thanks to the abusive potential, the blood, and the unstablity of the magic, Dexians quickly decided that all blood magic and all things using blood were risky.
Nuts and Bolts Blood magic can be detected via either people who can see ley lines (of which there are very few in Dexia) and via a simple spell that doesn't tap ley lines, but does poke them into glowing a little and becoming visible. It is possible to mistake real ley lines, if they are unstable enough, for blood magic ley lines, but blood magic ley lines are weak enough that they don't really look the same. Due to the unstability of real leys in Dexia, it is not possible to hide blood magic via setting it up near ley lines.
There's three main ways Dexians use blood magic. The first is to use it the way it was originally used - a passive spell that applies the end result to an area. The second is to use a person as an energy source, and as the target. The third is to set up some sort of energy source (either a person, a ley line sort of battery absorbing emotions from the area, or a combination) and then to target the spell via marking someone with blood or cutting them.
While Dexians don't really understand it, blood magic is not automatically evil. It is powerfull since it can do really any class of magic so long as the caster understands how to use it. A healer is limited by their personal magical stores and their healing skills and whatever other talents thay have. A blood mage is acually just creating a battery that any mage can tap.
The most major drawback of blood magic is the fact that it can be very addictive (since the caster tends to get a power boost by using the spells) and very easy to abuse.