Death of Superman graphic novel was pretty damned dark and high quality too. Some might also consider Action 775 a little dark as well. I'm sure there are others but that's all I care to recall right now.
Oh yeah, the Superman/Batman arc with Supergirl and Darkseid in it could be considered "dark" as well depending on what your idea of "dark" is. It's been a while since I read it though so maybe I'm remembering it wrong.
__________________ Darwin's theory of evolution is the great white elephant of contemporary thought. It is large, completely useless, and the object of superstitious awe.-Dr. David Berlinski, Philosophy
Most people believe Evolution not because they themselves are dumb, but cause they trust the "experts" who are feeding them evolutionary fast food, and so they don't bother questioning whether or not it's true.