Armand was in love with Desiree. He fell in love with her at first sight, like a pistol shot, the story tells us.
"Armand Aubigny riding by and seeing her there, had fallen in love with her. That was the way all the Aubignys fell in love, as if struck by a pistol shot. The wonder was that he had not loved her before; for he had known her since his father brought him home from Paris, a boy of eight, after his mother died there. The passion that awoke in him that day, when he saw her at the gate, swept along like an avalanche, or like a prairie fire, or like anything that drives headlong over all obstacles." (Chopin)
Therefore, once he was in love with her, he was consumed with the idea to marry her. He did not care about her ancestry, or her background.
Because Desiree was adopted, and her biological origins were unknown, it is logical for Armand to assume that the reason the baby bears African ancestry traits is due to her biological family connections.
Since Desiree's ancestry is not known, they could not rule her out as being part African, even though she does not look African. There were people who were just 1/4 African who did not look African, but carried the ancestry in their genes.
Perhaps Armand knew about his African American origins before he married Desiree. Although he did not show any obvious signs of his mother’s ancestry, he knew that when he had a child this ancestry could be apparent. Since he was ashamed of it because he was a racist himself, he actually preferred to marry somebody whose origin was unknown so that if they had a black baby , his wife would be held responsible and he would be able to continue hiding his real identity. This interpretation portrays his cruel, coldly scheming and profoundly deceitful nature.