i can see where you're coming with Opeth, in some of their earlier albums they have a black-ish tinge too them... but not really with Deicide at all...
they have added more melodic (almost synonymous with radio-friendly in metal) elements, with recent albums, to their music. in essence what you have is a watered-down form black metal. black metal was never intended to be listener friendly, really. kinda like what happened to punk/hardcore.
...do you know what an inverted power chord is?