Although not all satanist practice it, a blass mass is ritual made up to look like a latin mass produced by the roman catholic church. According to the middle ages a black mass was done during a witch's sabbath. The black mass was not just one ritual, it was done to say prayers to Satan and spread blasphemies towards the christian God. it was said human sacrifices, atrocious deeds, and sexual acts were done during a black mass.
The Black mass wasn't just a Satanist thing though and was done throughout the occult scene. Aleister Crowley made something similar called a Gnostic Mass, and took some variations from the black mass.
It was said during this time it people could summon satan and perform sexual acts. It was thought that witch's would sell their soul to the devil in exchange for their fleshy desires.
Modern black masses are not as blasphemous as people thought they were like back then. Of course they have similarities, but people do not do human sacrifices or drink blood, people who do this are just insane and are not real satanist.
Beginner Satanist usually perform a black mass to dishonor the name of God and initiate themselves into satanism. Though this isn't necessary to become a satanist, all you really have to do is be one and that is it.
As I said before not all satanist perform a black mass, but if you wish to, it is your choice. I hope you enjoyed this thread and learned a few things.
The concept of a black mass was originally a fabrication made based on the idea that everything "witchcraft," after it had been declared to necessarily be working with Satan by the government and over-ruling church of the time, was done as a sort of opposite to the Church. It was used as part of the justification to make any working of miracles or wonders (as many believed that miracles could only be truly done by God, and anything else must be lesser, and was thus called a wonder) illegal outside the bounds of acts by clergy.
Rumors of ritualistic sexual debauchery, human sacrifice, and more were added to make the populace afraid of the claimed sins committed, steering them to cling more steadfastly to the government-mandated Church.
Later on, some groups did adopt this concept, though originally it was most definitely not the case.
In truth, after Christianity had taken hold so strongly, most workings in magic and witchcraft in western Europe were done in the name of God, or Jesus, or the Holy Trinity. It is this era in which Christianity and witchcraft were most associated. Some elements of older traditions remained, but most were so intermixed with Christian concepts that the older traditions themselves were almost completely lost.
It is because of this that Latin (and Latin-sounding words, and the concept of magic languages and words) is so closely associated in fantasy tales of witches. People would learn scriptures in their "proper" Latin, and use them the way so many do today, such as praying the Psalms or evoking names of God as two easy examples.
Of course, even though Jewish people were not very common, part of why they were feared was because of the mysticism which was popular among them. This also lent fear of Jews, and many rumors that they practiced so-called dark arts and witchcraft, when really Jewish mysticism has long been about an experience with the Divine, and depending on era and tradition of mysticism, predicting future events and the end of the world (eschatology).
The use of Hebrew in their rituals also lent credence to the idea of magic words and strange languages, though Latin seems to have still dominated the cultural ideas of what magic "sounds like" (for lack of a better way to phrase it briefly).
Just piggy backing on what prsona explained, the definition of a mass is the gathering of many people, what there purpose may be can be called differently. So being called a Black Mass was simply a method to bring out negative connotations to those "Non-Holy" meetings. Of course because Black was considered a Dark and Foreboding colour In the West and Most of Europe. As a side note the meaning of colours vary from religions and regions of the world.
