Introduction: The so-called "Black Mass" (possibly, at least initially, a literary invention) was to be performed by an ordained priest—Catholic, Anglican, or Episcopal—(in later descriptions he was supposed to a de-frocked priest). It was an ordinary mass with magical ceremonies added, and was designed to pervert the power of the Eucharist to some magical end. It necessarily required belief in the power of the priest in Transubstantiation.
Description: The Walls of the chamber should be hung in black drapes; the altar is covered in black drapery; behind altar is a black curtain with a white cross on it. The woman—originally supposedly a virgin but later she was a Priestess—lies naked across the altar at right angle to its length, on her back and her knees drawn up. A pillow supports her head and she holds black candles in her outstretched arms. A cross is placed between her breasts and the chalice between her thighs. The priest is positioned between her knees.
Theory: The living body of the priestess forms the altar because the worship is of Creation from which all life originated.
As can be seen from descriptions derived from multiple sources, the black mass evolved from some use of the Christian ritual "perverted" for magical purposes, toward a more Pagan ritual of Nature worship. Whatever historic reality there may have been, the fiction probably led to actual enactment but information about it may be locked in Church archives. Later adaptations by modern Satanists were probably more theatrical than actual; and subsequent Pagan and Sex Magical enactments involving priest or magician and priestess or assistant are better studied under their own subject listings.