We have discussed briefly your religion before. I know it doesn't always mean evil... I mean, you know I think you are barking up the wrong tree. The truth is at some point those who are wicked - I'm talking - WICKED - will be wiped from the face of the earth.
What you are trying to imply is there is good magic (white magic or white witch) and bad magic (black magic or black witch).
Some define black magick as the intent to inflict harm on others and, according to the same belief system, it doesn't work - it karmically backfires on them, yielding no damage to the intended target. Thus indicating, at least to those who follow this line of thinking, there is no such thing as "black" magick whatsoever.
However, the word magick, much as I indicated with the word witchcraft, is simply a word. Just as the word fair has numerous meanings, so too does magick. In my view, life is magick. I have a few plants at home which I tend to frequently and the simplest things about life can amaze me. Watching my plants grow and flourish when watered properly, or how they bend towards the light. To me, this is magick (other people might simply call it gardening but we use words to convey emotion and that's the point I'm trying to make here). But I also consider the invocation of one's god or gods to be magick, by definition (my definition). Thus, prayer is magick. Thus, being Christian is no less "magick" than any other religion. Indeed, there are multiple pagan aspects of Christianity (such as the reverence of saints, the holy trinity and even simple acts such as the lighting of a candle).
Performing witchcraft, performing rituals, are a psychological way of reinforcing what one believes, a way of solidifying one's feelings. If I believe the rock in my hand represents my relationship with the land, and if I then plant that rock in a place which is special (sacred) to me, I am simply reinforcing this belief, this bond. There's nothing black -- or for that matter white -- about it. Arguably speaking, I don't even have to use the word witchcraft to describe such acts. I could very well call it psychological ceremony. But one has to appreciate the use of power words. Look at the level of vehemence and emotion which some respond to when the word witch or magick is used. To me, this is also magick (or some could just call it emotion - I prefer the more poetic route).
Where does white and black magic come from? The same source. So how do you take what is considrered pure evil and make it look good then associate God with it?
If by God you mean the Christian god, that cannot enter into the picture for a practitioner who does not share the same faith as you. It's like calling a Wiccan a Satan worshipper when they don't even acknowledge the existence of Satan to believe in him in the first place. But, all things being equal, just as someone might see these people as Satanic, there's nothing stopping them from seeing the other person as the Satanic one. For example, crusader knights in Medieval Europe called their arabic enemies barbarians. Ironically, the arabs called their European enemies barbarians as well.