Well, you can have awful things happening in a story, without it being grim dark and bleak. It is all in the tone, and the attitude of the characters. Yes, grim dark characters can want to achieve things, but there is never an upbeat moment, or humour - I don't mean everyone cracking a joke - though that should happen because it does in real life - look at the Blitz for example and all the up yours Hitler humour that went on in London while it was being bombed to heck - but more the kind of commentary on life kind of humour -usually quite wince making.
A rather dark but not grim dark story is for example T Kingfisher's Clocktaur War duology. She is just too sharp a commentator and there is underlying very dark humour, so I don't find it grim dark. (She also writes horror by the way.) For the record I am not a fan of sparkly rainbows and unrealistically successful endings either.
I think the moral greys of the world can be written about without unrelenting gloom and because I react to tragedy with black humour, and push back and look for the positive, reading a book where none of the characters do that feels not just depressing, but to me that feels unrealistic. However I do know that my dark humour annoys some people who think humour in tragedy is out of place. For example I spent my mother's funeral being aware of how funny she would have found the midnight blue plush lining of the stretch limo, the incredibly tall, bony, elderly, sombre undertaker in a frock coat and that he arrived at our house with a glossy catalogues of coffins. All through I felt she was sitting in the corner giggling. I kept this to myself as the rest of the family did not share that sort of sense of humour that mother and I had.
