Blood Magic is magic powered by blood and the Kansen. In Rokugan it is generally considered to be bad and evil, and its use always leads to Taint. Originally, Blood Magic was not dangerous, but since the time of Ichiban it has been corrupted and always leads to Taint.
Some of the people we have met from Chun appear to use some form of blood magic but are not tainted. Possibly they are using the uncorrupted form that was around before Ichiban.