Why doesn't Desiree realize her baby's skin colour as soon as he is born?"Desiree's Baby" by Kate Chopin
Having been adopted by the Valmondes--whose name ends with the French word for world, by the way--Desiree has lived a life within the shelter of this family's home. Because the infant has such light skin, she does not wonder about it, as his little features would not indicate any racial differences at this point since they are so small and probably similar to whites. It is not until a quadroon (a person who is one-fourth African) is fanning her child who is now older that Desiree notices the similarity between her baby and the woman. Since her baby, too, is a quadroon, his hair and features may easily appear to be more white than African, and it is only by comparing him to a slave that Desiree arrives at her realization.
Of note, too, is the fact that New Orleans was famous for its quadroon ball attended by the French male aristocracy of the city, and many of these young women, the children of Creole plantation owners who came to the city to do business and who had African mistresses, were very beautiful and fairly numerous. So, it is understandable that Armaud believes that Desiree is the one responsible for the child's African genes.
In Kate Chopin's classic short story "Desiree's Baby," Desiree refuses to see skin color because she takes it for granted. She has prided herself as white and privileged her whole life. Her mother and father instilled in her the color code that "white is might" and "white is right." She was brought up to believe that the upper class whites held dominion over the slaves. She tells her mother:
Armand is the proudest father in the parish, I believe, chiefly because it is a boy, to bear his name; though he says not,—that he would have loved a girl as well. But I know it isn't true.
She's more focused on his gender and name than his skin color. Color is not an issue: Of course he's white!
Another explanation is that the baby's melanin (pigmentation) does not reveal itself right away. Mulatto babies sometimes are born very light-skinned and then take on color later (as in the baby's case) or not much at all (as in Armand's case).