In my experience, the most common name for a dark version of some media, especially fantasy or science fiction, especially film, is the gritty reboot.
This is the term used by Know Your Meme and Urban Dictionary, the latter of which defines it as
Re-starting a film franchise from the very beginning and making it a grittier, more realistic version that will appeal to adults.
This term is used widely; a web search for the term immediately brings up numerous articles discussing merit or lack thereof for given reboots, or for the very concept, or even for the very term itself. GQ magazine ran an article on the history of these reboots.
This term is fairly recent—the Know Your Meme article points to 2009 for its coinage. I suspect that it’s somewhat unlikely to stand the test of time—right now, gritty reboots are very much “in,” having exploded into popularity most notably with Batman Begins. They’re new, they’re hot, Hollywood is obsessed with them. But audiences may already be tiring of them—and in the case of at least some choices, ridiculing them. The GQ article, for example, was prompted by the announcement of a gritty reboot for The Powerpuff Girls, which as the name might suggest, is not a property particularly ripe for this treatment. (Then again, I have heard good things about the gritty reboot of Sabrina the Teenage Witch, a choice I had found about as surprising, so who knows?)
The point is, sooner or later, there will likely be a backlash against over-gritty rebooting, and we’ll see less of them done, and then at some point thereafter, it will be “safe” to make them on occasion, hopefully reserved for a particularly deserving re-imagining. It’s not clear that we’ll still be calling them “gritty reboots” at that point, though. If they become less common—particularly if there’s a distinct lull in which few if any are made for a while—we may no longer have a term for them and then if they come back as a cultural force, we may well coin another term for them. After all, as the GQ article discusses, the concept of a gritty reboot considerably predates the term “gritty reboot.”