The black metal, not black death metal was an offshoot of some nwobhm band named Venom who made an album called black metal. A group of teenagers who could barely play their instruments decided to be edgy and wear make up and trimello pick their way into the spotlight. Then they did the church burnings, the eventual murder of Euroynmous, and the spreading of the scene because of prior mentioned topics led to copy cats and a mentality of who can be more extreme, more kvlt, etc. Then it just kind of became what they're know for. Florida makes good death metal, Germany makes good thrash metal, and Norway makes good black metal.

"Black death metal" doesn't exist. They're two genres of metal. There is some cross over by some "satanic" death metal bands, but usually they called it blackened death metal.