As far as I can distinguish, the general songwriting is different for fully developed and matured black metal by 1994 and Death Metal which finished taking shape as a genre even earlier.
Death Metal originally focuses on that stream of riffs in a more-linear-than-circular song structures which blend into each other but present the listener with a variety of details. It is also highly rhythmic, the drums being an essential part in the creative process.
Black Metal, on the other hand, focuses a lot more on melody and harmonic progression than on the complexity/twistedness of ever-changing riffs. Riffs can and should be engaging but they do not take the spotlight like they do on Death Metal. The drums are more of a constant background, like an flowing river on which the tremolo-ing or riffing guitars carry the underlying melody. Chords sound big. Some people think this is only true for the so-called Symphonic BM bands, but in truth it applies to all genuine Black Metal. Even in more "brutal" Black Metal albums like Immortal's Battles in the North this is true yet not evident because you need to dig under the layers of noise.

IMHO, "Blackened" Death is just Death Metal with less variety. Although I like some bands of this style I do think they are not as strong as pure Death or Black Metal bands.