At first glance dark writing seems like an obvious concept. If we attempt to define it though, things get shaky. We can start off by assuming "dark" refers to a mood or theme. The word "dark" itself, besides addressing a lack of light refers to something characterized by gloom. According to another definition, dark, when used as an adjective can also refer to something threatening. Something concealed and mysterious. Or it can be something morbid or grimly satiric.
A lot of writing can crowd in under this umbrella. While many works of modern fiction have dark elements, perhaps it becomes dark when that is one of the primary intents of the author. For surely, while Harry Potter has dark elements, no one would ever consider it to be dark fiction. Nor would we consider the likes of Tom Sawyer to fall under that shadow, although Twain has some scary moments in the book.
I write a good deal of dark literature. And it is a deliberate process. I want to upset people. I want to give them something they'll remember, not because it's good writing, but because they are home alone, it's raining, and they keep trying to remember if they've locked the front door.
I believe the key to writing dark literature is to start with the true and the mundane. The more fantastical something is, the less impressive the fright and less disturbing. That's not to say that monsters and Halloween "thingys" can't be scary, but the more they attach themselves to the "real", the more effective they are. Consider the Frankenstein monster; horrifying to people around the turn of the last century, but less terrifying now because of the advances in science. However, Shelley's themes remain intact:the idea that those who meddle in the wrong closets of nature will bring forth tremendous consequences. Frankenstein's monster now becomes a metaphor. Biological warfare, anyone?
As for the mundane...what could be more mundane than a house in the country where a family sits in front of a television set? Ask Jerome Bixby, author of "It's A Good Life", the tale of a town held captive by a twisted little boy who can alter reality with a wink. It remains one of the scariest "Twilight Zone" episodes ever seen. What can be more mundane than the shy, nervous motel manager in Robert Bloch's Psycho? What's more mundane than the soft and well-spoken Hannibal Lecter?
The most frightening monsters are those that wear the human masks; those that look back at us from the mirror.