Judging from your list of keywords, you've pretty much got it covered. "Dark" to me means many things. Sometimes it's in the overall mood of the fic with themes like death, destruction (physical, mental, emotional), loss of family/friends, evil (another Voldy-thing rises from within the ranks of the Death Eaters, for example). A post-DH world in which Voldemort did triumph and all that that could entail, from slavery of Muggleborns/halfbloods to acts of depravity. I've read some stories like that and they've been chillinggggggggg.