I don't know if this is close enough to what you're looking for, but I love The B Team (http://archiveofourown.org/works/686799/chapters/1260722) by twistedingenue.
This one has a Darcy who is morally grey, with worries that she might be getting darker. It's a team fic. Natasha and Clint wind up in a parallel universe where all of the main Avengers (including the parallel versions of them) are dead, so Darcy, Jane, Pepper, and Rhodey are carrying on the work as best they can. Darcy has become pretty hardened by the burden she has had to take up. It's a fic about fighting through darkness more than a truly dark!fic, but it might scratch that itch well enough. Also, so many awesome ladies being awesome.
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This one has a Darcy who is morally grey, with worries that she might be getting darker. It's a team fic. Natasha and Clint wind up in a parallel universe where all of the main Avengers (including the parallel versions of them) are dead, so Darcy, Jane, Pepper, and Rhodey are carrying on the work as best they can. Darcy has become pretty hardened by the burden she has had to take up. It's a fic about fighting through darkness more than a truly dark!fic, but it might scratch that itch well enough. Also, so many awesome ladies being awesome.