Why is there an "air of mystery among the blacks" and frequent visitors to L'Abri after Desiree's baby is born?
When the baby is about three months old, Desiree awakes one morning with the sense of "something in the air menacing her peace." She has been on a honeymoon of sorts both with the baby and with Armand. Armand has been less harsh with the slaves and softer toward her. Now something is obviously wrong. At this point in the story, Chopin keeps everything ambiguous; in fact, ambiguity is a hallmark of this story. Based on what happens later, however, readers can infer that the "mystery among the blacks" means that the slaves have noticed that Desiree's baby is developing African facial features. This would be astonishing to the slaves. Mixed blood children were common on the plantation; in fact, La Blanche's boy, who comes in to fan the baby, is quadroon, or one-quarter black. But those children have black mothers and white fathers, born from slavemasters abusing their female slaves. If Desiree's baby has African heritage, then either Desiree or Armand must be part black, or Armand is not the father. Any of those scenarios would be momentous.
Not just the slaves recognize the black features of the baby. Armand has to have noticed as well; he calls in "far-off neighbors" to weigh in on the situation. They apparently confirm that the child is black and that this is a serious problem for Armand. This results in a "strange [and] awful change in her husband's behavior" toward Desiree. Armand now knows that either Desiree, whose heritage is unknown, is part black, or he is—a possibility he cannot entertain even for a moment. Eventually Armand tells Desiree he wants her to go. She complies, and Chopin leaves it ambiguous whether Desiree believes she is of mixed race or whether she understands that Armand is and she must sacrifice herself for Armand's reputation.
In Chopin's "Desiree's Baby", the use of inference is very important. Readers are first cued in to there being issues with Desiree and Armand's baby when her mother arrives and talks about how much the baby has changed. Active readers know here that something is out of sorts.
The visitors to L'Abri are coming to see what Armand's slaves are rumoring about. The frequent visitors are coming to see the baby based upon rumors that something is just not right about the child. This information is implied, not stated.
By the end, readers come to know that the child is part black. Slaves, during this period, would recognize black characteristics in a child. They would then leak out the information to others that there was the potential that Armand's child was part black. This could ruin both Armand's names and the respect others held for him.