Grace's Hard Lesson is good. So is Second Place by Terri Madison.
Some other favorites are - A Film For A Few Friends by J. Wheatfield
- Blackmailed by Switch Blayde
- Blackmailed by the Brother-in-Law by Stormbringer
- Blackmailed Into Perversion by Linda Jean
- Blackmail with a Twist by sweetcakes
- Detention by MD James
- Her Husband's Boss by George VI
- Karen's Descent by Terri Madison
- Kelly's Conversion by notmystory
- Pet Teacher & Bringing Down Pet by Deva and Parker
- Robin's Descent by Danger-Bird
- The Secretary by A. G. Thomas
- Stacy's Senior Year by Parker
- Wedding Day to Remember by Linda Jean

There are many more blackmail stories, but these I feel are the best.