Hmm...some strains have a genetic predisposition to produce purple foliage, but black? That doesn't sound like genetics. On the other hand, colder temperatures can cause some abnormalities in purple strains...like causing the plant to turn a darker shade of purple. Maybe this is what's happening to yours.

Do you know what strain it is? Also, if you could provide pictures, it would certainly help others to ascertain what's happening.