Yeah, MF are simply coming from very different backgrounds than most first wave bands - they were into prog rock and 70s metal/hard rock like Priest, Uriah Heep, Led Zeppelin and Sabbath meanwhile most first wave bands took influences from Motörhead, Venom, Discharge etc. MF definitely were able to create a very evil atmosphere (think 'Satan's Fall', for instance) and had riffing that clearly influenced black metal (the open minor chord riff on 'Into the Coven').
I actually never thought that Emperor were influenced by MF but you could very well be correct about that - I've got listen to their material today because it's been over a year since I last listened to them. CF did have some MF elements in their music but I think they were mostly a band somewhere between Sabbath and Venom: quite middle-tempo with tons of Tony Iommi-type riffs but filthy, punky and aggressive as well.
Sacramentum and Dissection are both BM, definitely, if you ask from me. Dissection do have a stronger death metal influence in their sound but even so, they definitely weren't a death metal band.