It makes you feel down to watch that show (or at least, it did for me). THAT is what makes it dark, to my estimation.
This seems to confuse "dark" with "depressing" or "bleak". I do agree that tone can affect whether or not something is dark, but I don't think it can be the sole determining factor. There's a lot of Mawaru Penguindrum I found really depressing to watch because it hit home pretty hard, and in many ways it's a very bleak fictional universe. But I don't think it could be fairly called "dark" in any way. Disturbing, perhaps, but there's moments of joy sprinkled through the show, and while the ending is sorrowful it has a certain happy quality. It's certainly not a dark story, just one that explores some disturbing themes.
Like... there's a comic called Crossed (I don't recommend it) that's extremely dark, but doesn't really have an oppressive atmosphere because it's usually written in the spirit of an exploitation movie. It's meant to shock, sicken, and titillate, but not to oppress. It's more a parade of the grotesque. Crossed is relentlessly, unquestionably dark, though, because it's about a universe where humanity has no future, no protagonist can possibly accomplish very much, and senseless cruelty lurks around every corner. To read it is only to see horrible things happen without end.
So upon some thought, I think hope is important because it indicates whether or not the protagonists have any real power to address their situation, and turn an oppressive status quo into a beneficial one. I think that's why, for me, whether or not an ending is happy depends on whether or not characters accomplish goals or got something they wanted. In a truly dark story, the protagonists don't have the power to change anything, and so the narrative will necessarily demand their failure. And the protagonist's failure will often have its own horrible consequences.
So what would save Transformers from being dark, for example, is that nine times out of ten the protagonists will still get to do something that matters and achieve some sort of meaningful goal. Any deaths that happen along the way are very sad, but they aren't testaments to the futility of trying to do anything. And I think dark storytelling must contain a certain element of nihilism. The protagonists must be helpless to do the things that any reasonable person in their position would want to do. To paraphrase 1984, a dark story is one where a boot is smashing a human face forever.
