This HaBO is from Judy, who is hoping to find this romance:

It’s a historical romance.

I remember a notorious madam us blackmailing all her former clients by sending a letter asking for money and threatening to put their name in the paper if they don’t comply.

I also remember that the heroine challenges the madam to a duel.

I loved the book, but can’t remember title or author.

This one sounds interesting! Does anyone know it?