What is the conclusion of "Désirée's Baby"?

The conclusion of "Desiree's Baby" is tragic: Desiree's child is dark-skinned, which implies that either she or her husband Armand have African ancestry. In the Antebellum South of the setting, to be part-black is a mark of shame. Armand, assuming Desiree is the one with black ancestry (since she was a foundling and no one knows who her biological parents are), essentially exiles her from his life.
Desiree is miserable and claims she wants to die if it turns out she is not fully white. Her adopted mother writes her a letter bidding her come back home with her child. After Armand makes it clear he wants Desiree gone, she takes her baby and walks to the bayou, still in her nightclothes.
Chopin makes it clear that Desiree is never seen again at Armand's estate, but the exact nature of her fate is unclear. Depending on the reader, one can assume she and her baby died in the bayou, or (less likely but still valid) she eventually reached Valmonde and lived there with her adopted parents in social exile.
When trying to rid every trace of Desiree from his estate, Armand ends up learning that he is the one who is part-black, as his mother was black. His pride and racism not only compelled him to drive away his wife and baby, but they have also left him humbled.

The conclusion to the story of "Désirée's Baby" is very sad and tragic indeed. Désirée has been abandoned by her husband Armand, who's discovered to his great consternation that his wife is part black. In this deeply race-conscious society the color of one's skin is considered absolutely critical to one's status. African-Americans are slaves, occupying the very lowest rung on society's ladder. That Désirée should have African blood coursing through her veins, even if it's only a drop, is a source of great shame to Armand. That shame is compounded by the fact that the baby to which his wife's just given birth—Armand's baby, no less—is also part black.
Having banished Désirée and her baby from the plantation, Armand sets about destroying her letters and other personal effects. But as he does so, a shocking secret is revealed. While reading an old letter from his mother Armand discovers that, he too, is part black. In that one moment, Armand's exaggerated sense of honor and pride, based on nothing more than racism, has been completely destroyed. Now he realizes, to his horror, that through his mother he belongs to a race cursed with the brand of slavery.