I don't think that Black and Grey morality necessarily applies to the protagonist or heroes in Noir. Look at The Maltese Falcon. Sam Spade wears the white hat. It may not seem like it for most of the movie, but by the end you can see that he was looking for justice (and not just revenge). He even turns down something like $5,000.
Anyway. What I'd do is steal from a movie or book. That should give you a nice set of NPCs and good backstory to work with. The tricky part is getting the PCs into the plot; when in doubt, use force and violence, because it's not something the PCs can simply walk away from. Once you figure out how you're getting the PCs involved in this mess, play the NPCs as real as you can and eschew the idea of set-piece scenes. Those scenes should form naturally. e.g. PC: "This guy has a key to a warehouse, let's check it out." DM: That's where they are bringing in the drugs. There must be some guards there; when the PCs get there they will have to deal with the guards.
You need to get some buy-in from the players. They need to engage whatever noir plot is going on. Try to put the PCs on the bad side of the law, or make the law corrupt, in order to prevent the PCs from turning the case over to the cops or the feds and walking away.