The Fade is the place one goes when one dreams, the realm of demons and spirits, and the source of magical power. Entering it makes a mage vulnerable to possession if they aren't in complete control of their power, which is why the Southerners fear magic so. Foolish of them, of course.
"Light" and "dark" are ridiculous descriptions with vague, arbitrary delineation. Of course, there are levels of blood magic that should be considered unforgivable.