having read only the title, I'd say too dark is when you can'[t read the character sheets or see the dice. Bam!
1) don't confuse sexual for "dark" or horror. "adult" themes may make people uncomfortable, especially sexual violence, but shock value isn't "dark" and it isn't quality.
2) Humour and safety are SO important to horror. You have to care about the family and their chemistry together before it matters that Freddy is eating the kids. Better example: Discworld novels have threats, but they're to these characters who, compared to serious novels like Wheel of Time or even Game of Thrones, are silly. However, in less than 300 pages Pratchett gets readers to care about these silly people: silly on the surface, but with a deeper explaination that makes its own sort of sense within the world of the book. Threats to that "ecosystem" if you will, matter.
3) Dark tends to focus on gritty issues, imho, but gritty in terms of realistic issues. However, they also tend to have a level of hopelessness involved. [sblock=spoiler for The Ring] The Ring, for example, has numerous people in situations that are just depressing IN CONTEXT: the parents who are old and lost their kids. That is huge, because you're tapping into the loss of their family life, and the idea that they're basically just growing old: no one will care for them when their bodies fail, they're just living out their days watching other people have lives.
The insane asylum is, sadly, used as a hell hole for the mentally ill (and the reason no one funds good institutions that WOULD be an Asylum, merely for the fiction trope. Grr, Arkham!). It's drab, there isn't hope, no one's getting better.
Contrast that to the protagonist: a mother trying to protect her son from the video tape.
Japanese culture has a lot of traditional value on having kids, on family. Tapping into the fear of that loss is frightening.[/sblock]
3) To be effective you also need a firm understanding of good, of hope. Sadly, people who like horror or make it nowadays are so far gone from reality that "good" doesn't have meaning. They like the twist of the knife so that characters are fodder for some horrorific critter who becomes the star. Remember how Freddy Kreuger (a pedophile!) gained folk hero(!) status in the 80s? As a kid I saw kids in Freddy masks, with t-shirts! He's a pedophile!!!!!!
Without a respect for a healthy family life, a sense of justice, and of sexual morality, we WOULD NOT have the horror classics that modern fans judge new films by. At best they're thrillers or torture films which, and I can't stress this enough, are NOT horror films. Horror relies on having a person who can be horrified by acts: there needs to be good in order for bad to have meaning. I can cut a steak, but it's not a horror act unless the steak is alive, screaming, and we the viewer actually see the harming of innocent steaks as wrong.
So to answer the title of the thread more seriously: dark is only existent as far as you can make light that it surrounds light. So far as you can make your players or readers care about the light.
Otherwise you're just making a torture film, at which point you don't have dark. No dark, just cutting steak. And hey, steak is just steak, so who cares?