For something to be dark, it'd involve a general bleakness in tone and subject, regarding a situation in which there is little chance for escape. It could apply to anything from aliens to zombies, to suburbia or degenerative diseases, although to make the aliens and zombies dark would require more effort than to just say "ZOMBIE ATTACK". I haven't read World War Z and don't plan to - that book seems too dark for me, as it involves the destruction of worldwide civilization. (On the flip side, Shaun of the Dead doesn't come across as all that dark, since there's clearly a good chance of getting out alive.)

I'd describe "darkfic" as something where there's no hope to begin with, or the most the characters can hope for is to end the story relatively intact. The first example which comes to mind is someone's homage to I Have No Mouth, and I Must Scream: the story starts out bleak, and only gets worse from there.