Now that I think about it, I do remember Karen refering to him as "ruler of the void" at one point (can't remember if it was before or after she is actually written in Serenity in her true self, but Ido remember this one piece of dialogue). So it was kind of specified, you just have to be damn attentive to dialogue itself and connect dots every now and again.
Soooo if I had to guess... (and I have to, for now) I'd say shade magic was probably never really linked to evil and/or negative things, at least not until dark writting began. Because Shade herself isn't evil and never was, she doesn't want to plot against the king or anything, she just wants balance. And since shade magic is ALSO focused on lust (I previously thought it was purely focused on negative feelings and thoughts), I imagine that it's origin also isn't necessarily linked to any form of "primal evil". But I can't quite imagine what it would take to create the opposite form of magic to fade, which by itself originates from something even more mysterious and could very well be involved.