What did Armand find out at the end?

Weeks after Desiree's death, Armand is having his slaves feed a bonfire. He places a willow cradle and other remnants of his marriage to Désirée on the pyre, and the last object to burn is a bundle of letters. Among the letters is an unrelated letter that came from the same drawer, which was sent from his mother to his father. In the letter, which Armand reads, his mother thanks his father for their love and thanks God that Armand will never learn that his mother has mixed blood.