That's a good age for Lloyd Alexander's Prydain books, though I'm not sure what counts as "overly dark". There's war, characters die, things are scary and sad in places. But they're also so beautiful and full of hope and belief that the hapless, unprepared individual can figure things out well enough, and... no spoilers, but Taran Wanderer handles the chosen one/secret prince idea better than any book I've read before or since.

On a lighter note, perhaps Noel Streatfeild? There are very subtle lesbians in Ballet Shoes but it really took me until adulthood to go "Hang on, why do the two lady doctors live together?" so it won't ping the radar of anyone who isn't looking very closely for it.