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.