first, define your term about "dark arts." every people here have different definition about the so-called "dark arts."
about your question "what is the darkest of the dark arts?" if it is the type of magick you are talking too, then there isn't.. magick has no color in general.. it is just how human mind categorizes things to have a clear definition of a term in their own ways..
but again, your definition about the term is highly considered..