Black Metal = Music that worships and/or cultivates Satan or Darkness
Death Metal = Music that worships and/or cultivates Death
There is not one single way Opeth has ever been black metal. Some people seem to get melodic death metal and black metal confused because of similar vocal style(this was in reference to the vocal style on the early LPs of Opeth, because now they obviously have a death metal style of vocal). I have, and always will just consider Opeth a progressive melodic death metal band (unless of course they change music (der) and lyric themes).