I believe that Chopin was making a statement about color and race, and the prejudice that came with it during the times of slavery. I also think she wanted to make it known that slave owners did sleep with their slave; and in those times sexual relations between people of different races was frowned upon, so it was kept a secret.
I think that Désirée was so easily blamed for the child’s skin color because she was the one whose origins were unknown. Armand knew before marrying Désirée that the Valmonde’s weren’t her birth parents, but at the time he looked passed it because he was so in love. “…the girl’s obscure origin. Armand looked into her eyes and