In the simpliest terms, all magic is the same and really had no color, but the media, movies, television and video games has made color magic very popular. In the media, black magic is portrayed as evil, white as good. Red is often seen as love magic, green as money or wealth.
Such a division of magic into colors is useful, you can color-code your own spell code, writing love spells on red or pink paper and make them easy to find, but in reality this division of color does not really exist, the 'magic' is the same for all these spells and rituals.
Real practitioners consider spells which fight against, or try to manipulate free will as black magic. Because these spells try to influence the normal life of others they can become pretty dangerous if they backfire. Free will is considered by some as the strongest force in nature so care should definately be taken when dealing in the dark arts.
The line between what can be considered back magic is fuzzy though. Love spells are rarely seen as black magic, but if the spell makes someone leave their spouce and travel across the country to be with you you are definatly messing with free will. However, a simple crush spell to get a date doesn't really mess with their free will.
No matter how you decide to define black magic, the following are generally considered as black magic spells: