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.
Thankyou for reading and HAIL SATAN!