I have read ALL of Sarah Monette's work under both her pennames - no, lies, I actually skipped the last two Iskryne books, which like The Cobbler's Boy she co-wrote with Elizabeth Bear. IMO Iskryne is her only grimdark work: the story is about humans who are psychically bonded to wolves and when their wolves have sex, their psychically bonded humans have to have sex with each other a la the dragonriders of Pern... except if you took out everything that made the dragonriders of Pern super iddy and just made it grindingly awful, which I'm sure is exactly what such a system would be in real life, but clearly the whole thing was only ever supposed to be id fuel so why not just let people have fun?

...anyway. I've never really thought of the Doctrine of Labyrinths as grimdark: to me it felt more like a deliciously OTT angstfic, where lots of bad stuff happens to the main characters because they suffer so beautifully, but the point is the beautiful suffering and not the ultimate dark hopelessness of the world or whatever the point of grimdark is supposed to be. It's very much of a piece with the aesthetic in The Goblin Emperor, just on the darker end of the same aesthetic spectrum.