Let's get straight to the first question everybody is going to ask: Mary Emmons, aka Eugenie Beauharnais, did have two children with Aaron Burr- the DNA is in on that one. The family story is that they married at some point after Theodosia (whose first husband first brought the then-enslaved Mary/Eugenie to mainland America) died, but there doesn't seem to be any documentation to back that up apart from said family story. (Scott seems to take the marriage as much more definite than I can find other assertions of. Is the fact that the children used his last name supposed to be a sign that he acknowledged them at some point? Mary was still enslaved by his family when those children were born; she used a last name distinct from her enslavers', but does that mean that the children definitely would have used her last name unless Burr married her?) As the afterward tells it, all else that is known about Mary is that she was probably born in Calcutta, that she came to the future United States by way of Haiti, and that by the 1790s she was living as a free woman in Philadelphia. (Her children are much more present in the historical record, the daughter having been part of an unsuccessful attempt at free African American emigration to Haiti after which she returned to the U.S., the son a prominent abolitionist.)
As a result of the thin historical record on Mary herself, most of this story as it regards her life is invention. Alas, what is invented is quite thin. I never felt like I had a sense of Mary as a person and not just a window into a time and a place and a condition of slavery. She exists in this book to convey a narrative, and that narrative is not new. And familiar though it is, it's disturbing, and not in a way that feels intentional. Although Mary does seem to enjoy Burr's company when they first meet, their sexual association begins with rape, and it's never entirely clear if the reader is meant to understand if she comes to love him, or if she decides to submit pseudo-willingly to being his mistress in hopes that it will help her win freedom for herself and her children, or if she merely submits for as long as she has to. In the prologue, looking back from decades later, she claims that he loved her (though whether or not she loved him goes unremarked on), but one sees precious little evidence of that in the main narrative of the book.
Given the first person narration, there's no reason why the heroine should remain such a cipher, or that it shouldn't take a stronger stance on the relationship that forms the heart of the book.
Overall: C