Dark fantasy is being applied to every genre from horror to romance to sword and sorcery, so is it applicable to vampire fantasy as well? From what I have found so far, the answer is yes. It seems that one of the defining elements, as in Gothic fiction, is a sense of foreboding or grim setting. I have also found that many authors of dark fantasy lean toward the protagonist being a monster or magical creature themselves and not the victim as in contemporary horror.
Personally, I enjoy stories that are told from an actionable protagonist, anti-hero, or otherwise active participant in the drama. That is, it is refreshing for a story to be experienced as the instigator rather than the victim or bystander, and though I realize this is not for everyone, I feel that is why The Vampire Chronicles by Anne Rice is such a phenomenon. It presents a totally new perspective about vampires, their world, and motives.
Vampire fantasy itself is its own subgenre of fantasy and supernatural or paranormal fiction, however, with the zillion different Amazon books categories, it has become more and more difficult to pin a book down into one genre. Maybe that’s a good thing. Maybe now that we have more specific subgenres, it will become easier to find the specific kind of fantasy you enjoy.
Vampire fantasy is evocative of the darker side of fantasy elements, like blood and death, but sex is always an integral part of any good vampire tale whether it is obvious or not. The mere act of blood drinking and the powerplay that occurs between a vampire and his victim is indicative not only of mystical power, but also physical and spiritual power over the victim. Since blood itself is representative of the life force and, some believe, the tie that binds the soul and body together, it is conceivable that blood drinking would be equated with darkness since this act is robbing the victim of their life force.
Dark fantasy may well become a broader genre, as much of the work by authors such as Dean Koontz fits into it nicely. Some people label dark fantasy as a mashup subgenre, as well it may be, but that makes for all the more interesting storylines when magic, politics, religion, and sex combine against a supernaturally brooding backdrop. It suddenly becomes poetic and moody and Gothic, which brings it full circle to writers such as Lord Byron, Edgar Allen Poe, and Joseph Sheridan Le Fanu. (My personal favorite is The Monk by Matthew Gregory Lewis.)
Perhaps dark fantasy is anything that is not easily explained by contemporary reasoning and that appeals to our fascination with the unknown. Vampire fantasy certainly fits that bill, as the vampire is an ever-evolving shapeshifter that may mirror the darkness we fear to face in ourselves.