DarkStalkers Resurrection and the Games on Demand version of Dark Souls are heavily discounted until June 4. You can pick up either game for $10.
The marketplace also has Ghost Recon: Future Soldier for $15, along with some discounted DLC for the tactical shooter. We realize that one doesn't have "dark" in the title, but it does have "ghost." Both DarkStalkers and Dark Souls have ghosts in them, so it makes sense, see?
PSN is running a DarkStalkers Resurrection deal of its own as well. Pick up Resurrection for $15 and you'll get a free download of the excellent Super Puzzle Fighter 2 Turbo HD Remix. PlayStation Plus members can get the same deal for only $10.49. This promotion also runs until June 4.