Why is Desiree suspected of being black?
Because their baby resembles the quadroons of the plantation, such as the "yellow nurse woman," Zandrine, and the little boy who waves a fan one day as he stands near the baby, Armand Aubigny decides that his wife Désirée must be part African.
With her origins unknown, since she was found by Madame Valmonde as a baby, lying by a great pillar on their property, while Armaud is of a prominent family, he is convinced that Désirée's origins are the cause of his baby's racial mixture. In addition, Armaud suspects Desiree's origins since his father had originally wanted Désirée's family history divulged.
Now, Armaud is incensed; he rejects both the baby and Désirée, responding to her insisting that her skin is white with the cruel remark that it is “As white as the skin of La Blanche (to whom the quadroon boy belongs). Desperately, after receiving a letter from Madame Valmonde telling her to come home with her baby, Désirée asks her husband if she should leave, and he replies curtly, "Yes, leave."
Moreover he no longer loved her, because of the unconscious injury she had brought upon his home and his name. She turned away like one stunned by a blow...
Désirée takes her baby and departs, and is never seen again. Some weeks later, Armand has a great fire on his property; all traces of Désirée are burned. However, he ironically discovers, among the letters from Désirée, a letter written by his mother that had been in the back of the drawer; Armand reads it. With great irony, this letter informs him of what he has never known: His mother herself had lived always in France and been "of the race that is cursed with the brand of slavery.”