A black mass is a Satanic ritual that usually takes all the elements of a mass of the Roman Catholic Church but in the other way. The most notable symbol is the use of the reverse cross. The so-called experts themselves can not said for sure how they are conducted as none of them never been involved in a black mass. The most rational explanation is that the Roman Catholic Church itself created a powerful alter ego to affirm its power (an unseen enemmie amongst us can be useful when he is fake and used as political pawn). It is true that since the Roman Empire became the Roman Catholic Church, it had constantly replaced symbols, places of worship, festivals and rites of the ancient religions (referred to as "pagan") by their own and demonized anything they couldn't use or convert (for example, the Druids who did not become monks were called "sorcerers" thereafter).
Media related with Black Mass
Devil's Due (2014)
The Ninth Gate (1999)
To the Devil a Daughter (1976)
The Satanic Rites of Dracula (1974)
Dracula A.D. 1972 (1972)
Taste the Blood of Dracula (1970)