I think that Pratchett makes a lot of sense with 'Headology' - That the more power you believe something has the more likely it is to have an effect.
This is one of the key points that is likely to be missed by most. A wiccan spell, for example, is basically a form of prayer, using your faith (and the diety involved) to affect change in your enviroment. I have seen the effects of such things, to the point where the odds of coincidence become very large, but that doesn't mean I can offer an explaination for how or why it works. I have my own beliefs about it, but that's not really offering any explaination.
I have also had experience of various eastern ideas and training in manipulating the enviroment or your body with your will, which was why I asked the question I did earlier.
I do also believe there is stuff out there that we don't know everything about. I really doubt black magic has anything much to do with it though.
Black magic is really just a term for forcing negativity on the enviroment, at the very least it's not good to harbour those kind of feelings towards other people and desire to act on them, irrespective of whether you believe in any of the above or not.