No one's ever provided me with a reason why we can't consider Mercyful Fate black metal only in the context of back then, though. They are clearly not black metal according to the nomenclature of today. Black metal bands all have clear stylistic distinctions from other genres, which make them "black metal". I realize that Mercyful Fate helped to CREATE these, but that doesn't mean they are forever part of that style of music in the context of a definition today. As Life Sucks said, definitions can change. I don't disagree that they are black metal by the standards of the 80's but I'm failing to get an answer as to how they can be considered black metal given what we know today about how it has developed. I can agree with calling Mercyful Fate, etc. proto-BM or first-wave black metal...but genres shouldn't go as far as you're implying they should if the point of them is to make it easier to classify different styles of metal and recommend people new music to listen to.
I understand what you mean; if in 20 years "slam death" was no longer what Nick and I had written about it being...we would be justifiably pissed. But that's just how things go. I think you have to kind of let go some things in order to make facts more accurate.
btw Mercyful Fate has a SHITLOAD to do with black metal. It's just that, by current definition of black metal as a genre, Mercyful Fate doesn't fit in there. It sucks and is lame cuz a lot of 3rd-wave bands playing "BM" are gay or whatever, but that's just how facts develop and how the general public of metal fans accept things.