Norwegian black metal truly came into its own as a unique genre with the releases of Burzum, Under A Funeral Moon, and Pure Holocaust. Both of the latter ones have bridges to the trash roots of black metal. UAFM has A Blaze In The Northern Sky, with its heavy Celtic Frost influences, and PH has the Bathory worship of Diabolical Fullmoon Mysticism. However, both those albums also lightly tapped the door to some ideas that clearly distinguished them from almost any material ever released in the 80s. When the time came for Darkthrone and Immortal to release their second albums, they completely opened the door to those ideas and perfected them, shaving away everything else. I'd say that there was a natural progression even from the early 80s to the early 90s.
I realize that there's a hole here, and that it is Burzum, which doesn't have a "bridge-album", but was rather an example of full fledged Norwegian black metal done on the first try.
As for Mayhem, well I doubt anyone can argue that there's not a natural 80s-90s progression there.