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..
Indeed, even things which would typically seem "good" can be evil. For example you could meditate upon demonic influences or use healing to keep alive a suffering person whose karma means for them to die.
These guys are right sometimes its all how its used and the intentions of the person using it. There are magicks out there that are both made spiritually and strangely on a genetic level to work with only wicked people.