There's a saying among the contemporary Pagan community that "black magic is whatever works, white magic is anything else." This stems in part from a misconception that black equals bad, white equals good, and that there are no gray areas at all. However, nothing could be farther from the truth.
Let's look at the word black itself, and figure out why it connotates evil. A big part of that is thanks to pop culture -- after all, in popular shows like Charmed or Buffy the Vampire Slayer, the good guys are the "white light" people. The bad guys are surrounded by darkness. Another part of the whole black = bad concept is because of people who can't let go of their Christian upbringing, in which those who are righteous are surrounded by the light of God, and those who are damned will dwell in darkness.
The problem with this logic is that darkness in and of itself doesn't have to be bad at all. After all, how beautiful is a quiet night out in the country, miles away from the lights of a city? Have you ever walked in the woods at midnight, embracing the comfort of the shadows? Darkness is what lies in the soil, below the earth, before a plant grows in the spring. It is the long nights of winter, when we are drawn into our homes to embrace our families and count our good fortune. It is the inside of the womb, warm and nurturing. Even the darkness of the grave, of death itself, may be seen as welcoming.
Once we accept that dark isn't all that bad, it's a lot easier to look at the concept of "black magic" vs. "white magic." Even if we replace the words "black" with "negative" and "white" with "positive", we're still in a bit of a pickle, and here's why: because it is the intent that matters as much as the action. In other words, if someone performs magic that others might see as "negative," but does it for what they believe is an honest and just reason, then is it really negative magic?
To do magic is to say that you want to bring about change in the Universe -- after all, if everything were perfect, there'd be no need for magic at all. Any magic capable of causing change is also magic that can harm, simply by its very nature. Magic isn't some Super Spooky Power that we have -- it's a tool we can use to precipitate changes. Any tool can be used for helping or harming -- if I have a hammer, I can use it to build a house. I can also use it to whack people in the head. It's not the hammer that's "negative", but what I choose to do with it.
Case in point: in the early nineties, a serial rapist was terrorizing the women of a coastal city in the Carolinas. His reported victims, over two years, numbered at least two dozen, including a teenage girl who later committed suicide. A group of witches got together one night, and did a working calling for this man to be stopped by the Universe. A couple of weeks later, the prime suspect -- who was later convicted -- led police on a high-speed chase and wrecked his car, nearly dying from his injuries. He has lived since then severely handicapped, but he never raped another woman.
Negative magic, or no?
There are people within the Wiccan and Pagan community who feel that any magic that affects other people at all is unethical, and they have the right to not perform any magic on, against, or for others. However, there are an equal amount of people who believe that change brought about by magic is acceptable, just as change brought about by mundane methods is acceptable. Chances are, the two camps will never agree, but what you can do, as an individual, is respect the beliefs of those who may disagree with you, whichever side you may happen to fall on.
Look at magic as a way to improve your life. You can use it to bring love to you, to gain financial abundance, to eliminate problems from your life. You can use it as a method of growth and self-empowerment. It can be used to help you fulfill your dreams, desires and goals. Can you use magic to help other people? Sure -- if they ask you to. If they haven't asked -- or if they've specifically told you NOT to do anything on their behalf, then don't.
Ultimately, only you can decide which forms of magic fall into your personal system of ethics. If you feel a particular course of action is wrong, then avoid it. If you feel it is ethically acceptable, and you're willing to accept the results of your actions, then so be it.