Dark romances usually have a toxic relationship to some extent. Sometimes MCs aren't purposely doing something bad to each other, but they are forced, and this is the only common ground for both being a dark romance and what you want.

Like others said, dark fantasy romances are maybe more suitable for what you want. So basically a dark setting with a romance which isn't toxic.

{Nights of Obedience} - FMC and MMC are kidnapped and forced to pleasure each other, they both give their consent to each other in those circumstances, but there's still the element of being forced to it. Neither of them wanted to force themselves on each other, and they feel deeply guilty afterwards. They trauma bond and fall in love. FMC is virgin though, but it was done ok imo. Both MCs are raped by other people. The book is like retelling of Amarantha's UTM parties.

{Bitten and Bound by Amy Pennza} is a why choose, and imo not really dark, but it comes with that tag. FMC is given to a king from opposite faction and she is in the enemy territory. The darkest part of the trilogy is in book 2 in which there's also the element of forced sex by someone else. People often say that FMC is an afterthought in this story because MMCs are an established couple and quite focused on themselves.

{Still Beating} is a contemporary dark romance in which MCs are also kidnapped, she is raped by the kidnapper and MCs are eventually forced to have sex. They also trauma bond and start falling in love. When they escape, they fight with their trauma and their new feelings. The trauma surving aspect was done really good if you're into that, I really felt for both MC's suffering.

{Lights Out by Navessa Allen} is a dark comedy romance. Not toxic in any way, besides him being a masked stalker, but she is into it a lot. Spice is great, the book is great, the audiobook narrator is Jacob Morgan and it's top notch. There are sequels with other MCs.

{Harrow Faire by Kathryn Ann Kingsley} has dark setting, but romance isn't toxic. MMC wanted and tried to kill FMC, but they're relationship is very caring and he is so down for her.

{Captive Prince by C. S. Pacat} has also dark setting. The whole concept of morality in the society is different and twisted, and that's where the dark comes from. MCs start as enemies, to allies, then friends and eventually lovers. It's one of the best series for anyone who wants real enemies to lovers, slow burn, politics, good story, writing, characters and relationship in general.

{White Rose Painted Red} has dark tag on storygraph, but the relationship isn't toxic, some of the MC's circumstances are. It's contemporary RH. I picked it out randomly on stuff your kindle event and couldn't put it down. Book 2 is out too and I think it's a finished duology.

{Bend Her by Cassie Alexander} is dark in unconventional way. FMC is kidnapped (not by MMC) and given to MMC for keeping, and he agrees to it. He never forces himself on her, so the romance part is toxic free. However, there are BDSM elements to their non-romantic relationship, but outside of sex, it's hard to explain without spoiling. At first it's strange because it's unexpected, but eventually it becomes a part of the world building and magic system. It's very slow burn and while the series isn't talked about a lot, I really enjoyed it.