Yeah I was a black/death metal sperg in the 90's/early 00's. I wouldn't say Nationalism is a corner stone of BM, moreso anti-Christian and Satanic/Horror larping. NSBM was more of a minor sub niche. While there were some Nationalist undertones with the Norse stuff, it wasn't really about Norse/Swede pride, but everything white people do that celebrates Euro culture without some nigger involvement is "nationalism" or "fascism".
Retard leftists I knew IRL years ago were saying metal as a whole is racist, probs due to Phil Anselmo nazi saluting once or something and I'm like yeah all the white metal kids sure hated that nigger Kirk Hammet and they just do that derp look when their woke shit makes no sense and they have nothing to add.