The idea of a tape is obviously absurd, but I suppose he was trying to update a concept for modern viewers. Blackmailing masters who had sex with enslaved women strikes me as highly, highly unlikely. The practice was so ubiquitous, planters's wives knew it was happening, which has sometimes been attributed to the extremely brutal treatment mistresses could inflict on young female slaves.
If Harriet Tubman had sex with her master, it was rape. On that point I disagree with the Ms. author. Since she was owned, she had no right to refuse his sexual advances.
Enslaved women, however, were sometimes able to use such relationships to their advantage. This had to be understood in the context of literature on slave resistance and with full awareness of the absolute brutality of the institution. Consider, for example, Sally Hemmings, whose long-term sexual relationship with her master, Thomas Jefferson, led to her children's being freed in his will, while the rest of his slaves were sold off to pay his debts. There are problems with taking out stories of resistance to tell to the general public because they can sometimes be interpreted as diminishing the brutality of the institution of slavery. Historians who write such works know of that brutality, but the casual reader or observer may not.
There is a story of an enslaved woman, Celia, who responded to repeated rape by finally killing her master. Her story has been chronicled in a highly readable, short book: http://www.barnesandnoble.com/w/celia-a-slave-melton-a-mclaurin/1102736801?ean=9780380803361
Documents from the trial are online: http://law2.umkc.edu/faculty/projects/ftrials/celia/celiahome.html
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