The world is basically full of opposites. According to the Chinese belief in the Ying-Yang, the opposing ends make the world a harmonious and balanced place to live in. It should be noted that there are two sides to almost, if not all, things.
Magic certainly has two ends, the white or conceivably good magic and the black or bad magic. Both are said to be powerful in positive and negative ways, respectively. Black magic however is not totally bad for it has its good side to help in the performance of good spells, like those spells for love. Harmony does occur for what one cannot do the other can. Black and white magic can become very good complements of each other too.