I'm sorry for the late answer, but somehow your email slipped into my spam box!
I have a lot of dark stories lurking inside me. It's not really possible for me to ignore them, so I tend to circle around them like a vulture, taking runs at writing something that pokes at the taboo from different angles.
I don't really feel there is anything I absolutely can't write. But I do feel that often, with the really dark stuff, the very darkness of it hampers me writing about it well. The subject looms too big and refuses to be bedded down well in the structure of the story. Although I've written some non-con, some under-age stuff, etc., I never thought I'd done a compelling enough job of it to take it via the 'literature' route.
I think the great challenges and triumphs for some of the writers that Amanda mentioned is that they took this very edgy subject, and wrote about it with such elegance, such humanity, such insight, that the reader felt safe to go there with them, because the writing achieved the reader's trust. Those writers took readers to those very morally ambiguous places, and could be relied on to examine the topic in enough complexity and humanity to leave the reader in a safe, if uncomfortable, place. That, in my mind, makes something good literature. And I can't really claim to have written about any of my darknesses as well as that.
I would say that one of the best short stories I've ever read in this category is Mike Kimera's 'Nadica'. It's incredibly dark, very erotic, and somehow you feel like the writer is an honest, non-exploitative, compassionate guide.
KB adds: I rely on RG for an edgy story. That makes me curious about what she feels is dark.