Shades of Grey Edit
Dark magic, often referred to as Evil, or Black magic, is a term used to define magic that is misunderstood, and not necessarily evil, or good. Typically, Dark Magic has some connection to darkness, The Night, death, or the dead, while Light Magic maintains the opposite. Black Magic is simply a much darker form of Dark Magic, with Death Magic being the only True Evil form of Dark Magic. For Example, Necromancy is Dark Magic. It's the act of bringing the dead back to life, and controlling them. This may be viewed as wrong, and it could easily be used for evil purposes, but it is typically used to commune with the souls of loved ones long gone, or to help people do things they can't do otherwise, like build/protect a town, clean the house, or Guard an Ancient Tomb.
Forms of Dark Magic Edit
There are, like with all Magics, many forms and varieties of Dark Magic. Of course, not all are suitable for daily use, or for use at all, but most can be reliable tools in any situation, so long as it's understood that they are safe ONLY when used properly.