@thesparkwithin no offence taken. as I said, you don't have to apologise for stating an opinion :)
That aside, I think of historical romance as a story written with the intent of being set in a older era. So a romance written today, set in the the 1860s is historical romance, but a book written in 1860 is not. That is just contemporary romance written in the 19th century. Am I making sense?
So I see Jane Austen's works as contemporary romance written in her time. It's become classic literature today, but ain't historical romance. Also I like to think of HR as anything set in a timeliness prior to the 1920s. Anything after that just qualifies as retro romance to me. There is some level of lived experience amongst our elders or visual and through print and visual media (movies, advertisements, video recordings, archival evidence etc) which makes it seem less historical to me.
I never took to historical romance tho. Can't seem to pinpoint a reason. Maybe it's the overuse of the template of a duke falling for some commoner, a huge manor and sprawling estates in the background or the age-old children of warring families falling love. Maybe I just stumbled upon the wrong kinds of books in the sub-genre and hence never enjoyed it.
Dark romance is any book that explores protagonists whose actions, in a normal situation, would be termed villainous (kidnapping, sex through non-consent or dubious consent, stalking, rape, obsessive controlling behaviour, bdsm elements), yet in the course of the story these characters get a happy ending with their "victim" falling in love with them. The said protagonist also doesn't suffer any judicial punishment for any of their actions. Basically one character has a significant dominance over another powerless/submissive one and both of them end up in happily ever after. That's how i define dark romance as. The lines are blurry tho. What's dark for one person may not be so for the next person. Guess we all make our own definitions