from:
http://www.themystica.com/mystica/articles/b/black_mass_the.html
there is no set Black Mass ritual, rather the ceremony is a parody on the holy Catholic Mass. One ritual is that it is performed in entirety, or in parts, backwards. The Mass may include inverting the cross, spitting and stepping on the cross, stabbing the host and other obscenities. Urine, supposedly, was at various times substituted for holy water, or for the wine. Sliced pieces of rotted turnips, black leather, or black triangles were substituted for communion bread. Black candles were used instead of white ones. A defrocked generally performed the Black Mass wearing vestments of black or a color of dried blood, and embroidered with inverted crosses, a goat's head (referring to Baphomet), or magical symbols.
The magical significance of the Black Mass rests in the belief that the Holy Mass involves the miracle of the transubstantiation, that is, the magical or mystical changing of the bread and wine into the body and blood of Jesus Christ. If the priest can affect this miracle within the Holy Mass, then, it is reasoned, the priest, or magician, could effect similar magic in other masses for other, usually harmful, purposes. The Catholic Church condemned priests who attempted to subvert the Holy Mass for evil purposes, such as cursing a person to death, as early as the 7th century.
One such famous form of the Black Mass is The Mass of Saint-Secaire, which is said to have originated in the Middle Ages in Gascony. Its purpose was to curse an enemy to death by a slow illness, which wasted him away. Montague Summers renders a colorful description of it in The History of Witchcraft and Demonology.
The origins of the current known versions of the Black Mass date back to the 14th century in France. This was the time when the Church was persecuting heretics.The Knights Templar, in particular, were accused of conducting these Masses and also other blasphemous rites in which they denounced Christ, worshipped idols composed of stuffed human heads, spit on and trampled the cross, and worshipped the Devil in the form of a black cat. Through accusations and trails the order was tumbled, but whether all the accusations were true still remains a mystery to many.