What is black/dark magic?
Magic has no color at all. Magic is the intent of the caster. Magic is a neutral energy found in nature, as spell casters, we charge this neutral energy into our body, and direct it on a particular goal. Thus making it positive or negative 'white' or 'black'. Dark magic can be anything that causes negative effect on someone. Hexes, curses, revenge spells, and plenty more.
What about this thing called the 'Threefold Law'?
The Threefold law states whatever we do, good or evil, it will return to us three times. some believe this, others don't, it's a matter of opinion. The Threefold Law comes into play more often when someone casts negatively though. It is as much dangerous for them as the effect on the person the spell was cast upon. Dark magic is very powerful. For a lot of people it is very draining as well.
What does Wikipedia say about black magic?
"Black magic has traditionally referred to the use of supernatural powers for evil and selfish purposes. In modern times, some find that the definition of "black magic" has been convoluted by people who define practices that they disapprove of as "black magic" Almost everyone could cast a "black" spell. You're not a bad person if you do. Lots of people who practice magic focus on both sides of magic.
I believe that black magic is a interesting thing, and is slowly forming to what media has advertised it to be.