Let me begin by saying that I offer the following observation to be framed in the HP fandom. I really don't get out much so I can't tell you what goes on elsewhere.
In the Potter canon, Good triumphs over Evil - certainly it makes sacrifices, suffers loss, and walks off with a black eye, but it does win, because Good is written to be intrinsically more powerful than Evil (Lily's love of Harry trumps Voldemort's most powerful killing spell, etc.).
Now in the fandom we like to turn things on their heads - so we create the Darkfic. The compass of the darkfic is the inversion of that which drives canon. In the Darkfic, Evil triumphs over Good, because it is craftier (see the general love of all things Slytherin), edgier, sexier, better-dressed, and all around more indie. Plots of darkfics are exactly the opposite of what you would find in the book: Harry fails, the Death Eaters win, Voldemort stiffs The Boy who Lived. The difficulty with darkfic is that it picks up where it should rightly end - with the victory. In canon, once victory is acheived, JKR will cue the end-of-term dinner and close the book. But in Darkfic, the author is required to establish the story on a foundation of presupposed victory. (An author could do as JKR does and just have Harry snuff it in the end instead, but that might generate too many "WTF?!" reviews.)
We begin at the end - the forces of Evil have triumphed - that is how we know we are reading/writing a Darkfic. But from a story point of view this is not a good thing. We can either watch the author spin ever-widening circles of angst for eighteen chapters, follow a "good" character's degradation to a point at which they will not only carry the Dark Lord's child but they will even let him name it Tom Marvolo Riddle, Jr., or we can see a DE post-game get-together complete with back slaps, orgies, and a bit of karaoke by Lucius.
By starting at the end, we have no useful direction in which to take the story, except the beginning, and that is too complicated a thought to be addressed in an LJ entry.
Now the Noirfic (same name, different language) is a different animal altogether.
Good and Evil still exist in Noir, but they are constructs rather than actual forces. They are ideas formulated by flawed humanity. In Noir, nothing is really good or bad. Everything becomes subjective. The stories will have opposing sides with one felt to be wicked but this is based solely on the biases of the main characters or narrator. For example, one might have a story in which the Death Eaters do a bad, bad thing, and then the Order of the Phoenix retaliates in equal measure. If our perspective is that of Evan Rosier, we might declare the Dark Lord's cause good and right, and Dumbledore's cowardly and low. In Noir, morality has no real meaning aside from being a handy touchstone to illustrate character divisions.
As we have seen, the appeal of Darkfic lies in the Saturnalian twist of Evil rising over Good. Noir fics focus on something yet again - by discarding the grand images of right and wrong.
We are left with not morality, but politics, not heroism, but survival, and in place of the epic, we have realism.
A Noir fic should be a gritty, realistic affair (yes, even in a universe where men go about on magic brooms). Its center is not in the struggle between Light and Dark, but between man and society, man and his own flaws, the individual and the world. The hero of the Noir fic is the reluctant everyman trundling along, trailing baggage and weakness behind. He follows his course not guided by moral impetus, but by an instinct to persevere. It's a world of "you win some, you lose some", where bravery is based not in a certainty of what is right but rather a gut reaction against perceived wrongs.
In a universe where justice is faceless, mindless, and soulless, the divide is noticeably absent and bad things happen. The nobility of the Noir hero is in his need to see the quest through simply because it seemed like the thing to do at the time.
It's a little bit like life, really. Funny, that.