The idea of black witchcraft isn't as universal as you might think, and many experienced witches may even tell you that it doesn't exist as a separate type of witchcraft at all.
Most people see the difference between white and black witchcraft to be the same as the difference between good and evil. And yes, that's the typical way of looking at it, there isn't some clear-cut line between the two. Really, very little that you can do is ever going to be all good or all evil, and the same goes for the way you cast your spells.
That said, black witchcraft is usually the realm where you cast negative spells on other people, including hexes and curses. Then again, less severe kinds of spells like banishings, bindings or other protection magick is sometimes classed as being a kind of black magick because it is influencing other people against their will. Spells that involve other beings (usually hostile spirits or even demons) would be called black as well. Though this last area is more "movie magick" than reality anyway.
Otherwise, spells get their power from you and the Earth, and you're not getting involved with any "things" that can hurt you. You're not opening some nasty doorway, no matter what movies may make it look like.
Now, there is also the concept of karma to consider. Not all witches believe in it. If you believe that the energy you put out will come back to you, then you might have something to worry about. But karma can be balanced by good actions as well as negative ones, so you can earn yourself some good energy to keep things in check. Do some good things to get some karmic brownie points.
The last thing to consider with black witchcraft is that you don't always get the exact results you are aiming for (a problem with white witchcraft too, I suppose) and when you are dealing with strong energy, you can have problems if things to awry.
So black witchcraft involves spells that are negative or harmful, but that is a really rough estimate and it can really be up for interpretation. But let's face it, sometimes serious circumstances require serious action, so there may be a place for these kinds of spells on occasion.