Anyway, a simple path of witchcraft can definitely be either white or black, and either one is open to you. But Wicca is a more complete spiritual path with its own ethical limitations. That's where problems with black magic can come up.
Of course, there are many different branches or traditions of Wicca and not everyone follows the same ethical roadmap. But basically, the overall belief tends to lean towards "harm none" which can conflict with the nature of black magic. How you interpret "harm" can be a big part of it too. You may also follow the Rule of Three, where your actions can come back to you three times. In this case, you may consider doing black magic and accept any future consequences.
Basically, Wicca has ethical restrictions whereas non-religious witchcraft does not. If you want to stick with the usual Wiccan ethics, stick to the less harmful types of spells like banishings, bindings or other forms of protection magick. These spells can help keep people away from you but isn't considered harmful to them.
If you are going ahead with stronger black magic, then you can try curses, revenge spells or even black magic love spells. Even if you don't feel that black magic and Wicca go together, sometimes circumstances can be unique and may call for stronger measures than usual. On that note, I'll leave you with one spell to get you going. Check out the links above for more.
This is a general spell to cause bad luck or misfortune to fall on someone. All you need is:
Draw a pentacle on the paper, fairly large. Write your target's name in the center. Carefully sprinkle a fine line of pepper around the outline of the pentacle. Once done, light the candle and let it burn for a few minutes to get the wax melting.
Hold the candle over the pentacle and let several drops of wax fall on the person's name. Ideally, enough to cover the words. Then let the candle burn for an hour before putting it out. Leave your pepper sigil in place where it won't be disturbed until your spell has its effect.