The Wrong Mother by Sophie Hannah
My rating: 4 of 5 stars
Not quite black, more like dark grey - but definitely psychological weirdness in the manner of Barbara Vine.
Imagine having a one-week no-strings affair with someone: you'd probably tell them all sorts of deep dark secrets, confessions you might never tell another soul. So what happens when you hear that man has now killed his wife and daughter but it's not the same man you met. Sally is naturally quite disturbed by this, not to mention that she's overworked and her babysitter can't take care of her daughter during an upcoming conference. At the same time she's investigating who she did actually meet, the police are investigating a possible family annihilation. Or two.
The narration switches from Sally's story to the police investigation, with the two colliding as Sally meets "Mark" again, and the police figure out who he really is. I have to admit, I was surprised by the revelation.
Definitely an author to remember, the next time I'm looking for a book to buy.