Grimdark I think of almost as an aesthetic choice. If I use it to describe a show, it invariably has poor lighting and dirty sets. The dark Knight rises is grimdark by my definition. Little Drummer Girl, which is about the soul destroying things spies and terrorists do to protect their people, isn't, because plenty of the cinematography is brightly lit and vivid colours. Blade Runner is grimdark. Hunger Games is not. Lots of the first movie take place in a forest!

My husband's theory is that grimdark originated in Warhammer 40k, where the first illustrator was only given a month to do the art for the entire rulebook. So he did black and white sketches of a dystopia. Making grimdark an aesthetic choice there - it's physically dark as well as dystopia.

I often but not always dislike grimdark because it feels pretentious. I get the sense that people writing Tragedies think their work is more Important than those writing Comedies. And I get the sense that people writing Grimdark feel it's more Important than Bright Dystopia. And I don't agree! Harry Potter and the Philosopher's Stone dealt with child abuse and classism and all sorts of serious themes without undersaturating the colours. If you want to go grimdark, you'd better have a reason for it, not just do it cause you can. I like being able to see what I'm watching.

The same applies to books, but it's easier to explain my opinions with movies.