Death Metal: I really appreciate the qualities of Death Metal, due to that fact that it supports some "non-divine" intervention of anti-human perspectives, emotions (hate, disgust, sorrow and ect.) Those are well appreciated at certain times depending on one's mood, if that might be the case. Also, some bands who baptized themselves in Death Metal, has a different point of view towards society, religious/irreligious aspects and what not. Other bands (also the Death Metal genre) might not express themselves as well as other bands do in certain lyrical themes but through their instrumental capabilities, doesn't necessarily mean it is "not a good band, or being nailed down as a supposed "poser band" or any other negative criticism" -- Fact is, we all have our preferences and what makes us "go" in Metal.
Black Metal: Black Metal is a popular underground genre, for the controversial lyric themes, anti-christian imagery, and obscure appearances. The spikes and "corpse paint" (as stated by Varg Vikernes) is there to show "their" petition against some "socially acceptable" means general things in life. Now, as for the music style; I think personally Black metal has potential, even if it's fused/mixed with other sub-genres. Some Black Metal bands are atrocious to the ear and others gives you goosebumps like crazy (think some folks might agree to this), again it is a matter of preferences and liking. Black Metal on it's own, can be fused with Death Metal and Death Metal can be fused with Black Metal. Black Metal just show their "hatred" and "feelings" more clearly than some Death Metal bands. Even portraying that they stand firmly in their belief (Satanism, Norse Mythology and so on) which is regarded as "special" because they believe in something that makes sense to them. We cannot judge that unless we can do it better. I think?
__________________________________________________________________________
Black Metal VS. Death Metal
One cannot compare an apple with a pear even if both is fruit. As simple as that.