A Black Mass is a ceremony supposedly celebrated during the Witches' Sabbath, which was a sacrilegious parody of the Catholic Mass. Its main objective was the profanation of the host, although there is no agreement among authors on how hosts were obtained or profaned; the most common idea is that they were profaned by means of some ritual related to sexual practices. Authors also disagree on which rites were performed during the ceremony. Some medieval writers believed that the host was replaced by a toad, a turnip or a piece of dry flesh, but most judges and authors believed that true hosts were given by Christian priests, who had made diabolical pacts, to the attendants of the Sabbath to be profaned by them.
It is not clear if the Black Mass was ever celebrated in medieval times; the works referring to them are lurid and unreliable sources such as the Malleus Maleficarum, and it may have served solely as a shocking act with which to accuse enemies.
Extract Source: http://en.wikipedia.org/wiki/Black_Mass