There is a 13th sign found in the solar zodiac. Ophiuchus, also called the serpent bearer, falls between Scorpio and Sagattarius. Not all astrologers work with this sign because the sun falls in this house for only 19 days. You can find an article about Ophiuchus at http://simostronomy.blogspot.com/2010/03/ophiuchus-13th-sign-of-zodiac.html
The idea of the "dark zodiac" was created by Dark Castle Entertainment for use in a movie about ghosts. While the concept is great from a fictional point of view it has little or no relationship to the actual practices of astrology.