When Désirée looks at her child and understands that the child is of mixed race, she confronts Armand, asking him what the baby’s skin color could possibly mean. When he enters the room where she and the baby repose, he does so “without noticing her,” and he goes about his...
business. When she speaks to him, she speaks with “a voice which must have stabbed him, if he was human,” though he responds “lightly” and “cruelly” to her, as if it is nothing to him to wound her.
A sincere belief that Désirée is of mixed race herself seems to be the only thing that could compel Armand to act in such a way. He doesn’t notice her, just as he would likely not notice or in any way outwardly recognize the presence of a slave, because a person with Black ancestry would be so insignificant to him. The fact that he can speak so cruelly to her and ignore her very evident distress though she is the woman whom he fell in love with like a “pistol shot” also makes it seem as though he thinks of her as less than human now. Were he aware of his own racial heritage, it seems unlikely that he would be able to treat her so terribly. Armand’s treatment of Désirée would appear to confirm his belief that she is the person of mixed race, not him.
Further, his mother’s letter, which states, in part, “I thank the good God for having so arranged our lives that our dear Armand will never know that his mother, who adores him, belongs to the race that is cursed with the brand of slavery,” adds weight to the claim that Armand did not know about his own heritage. His parents obviously kept it a secret from him, and now he seems to want to keep it a secret from everyone else, and this is why he burns the letter. If he knew about the letter before, it seems likely that he would have burned it long ago.