Definitely dark, this little book is well-crafted, if (honestly) ultimately unfulfilling.
Stephen King apparently said “No one surpasses Ruth Rendell when it comes to stories of obsession, instability, and malignant coincidence,” and indeed all the characters (but one) are either obsessive, unstable or malignant (or a combination of all three). Whereas it is common in certain types of fiction to be frustrated with a character’s bad choices, that only happens if we have established some sympathy for the characters. I’m not sure that happened in this multithreaded plot.
But you may have a different opinion. Ruth Rendell, Dark Corners (2015). [amazon link]