I'm not sure exactly what you mean by darker, but I'm guessing you want Regencies where that have a high angst quotient.
Personally, I love my Regencies with a big serving of angst. I would highly recommend the following books/authors:
- Sherry Thomas, especially Not Quite A Husband and Private Arrangements. Time period on these is Victorian rather than Regency, and big bonus, they are $3.99. Both books made me cry.
- Anna Campbell's first two books, Claiming the Courtesan and Tempt The Devil. I'm not wild about her later books, but these were really good.
- Anything by Elizabeth Hoyt, but I especially like her Princes trilogy. She writes in the Georgian period, rather than Regency, so late 1700's
- Laura Kinsale - Flowers From the Storm is a classic dark regency, but my favorite of hers is Prince of Midnight.
- Anne Stuart's House of Rohan trilogy
- Ashley March - Seducing the Duchess
- Judith James - Broken Wing (mid 1700's I believe, and features a hero that worked in a brothel). Plus it's only $3.82 on Amazon.
- Delilah Marvelle has some very interesting plots and deals with heavy subjects. I've only tried one of hers though.
That should get you started.
Oh, and these are all large publisher books, not self-pubbed.