While the ironic ending of Kate Chopin's "Desiree's Baby" seems to indicate that Armand's ignorance of his mother's ethnicity, there are other indications that he has known, or at least, suspected.
1. After Desiree turns eighteen, Armand Aubigny rides by and is smitten by her. However, the narrator says,
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.
Now, if Armand were eight when his mother died, it seems logical that he would have noticed that she was black.
2. Chopin writes that ever since Armand is the master and not his kindly father,
Young Aubigny's rule was a strict one, too, and under it his negroes had forgotten how to be gay, as they had been during the old master's easy-going and indulgent lifetime.
Often when someone resents something about himself, he will be cruel to those who resemble him in some way such as character or person. Armand's behavior to towards the slaves indicates such a cruelty.
3. The narrator states that marriage, and later the birth of his son "had softened Armand Aubigny's imperious and exacting nature greatly," so, perhaps, Armand has been concerned that the baby would display traits of a negro; but, realizing such is apparently not true, Armand is relieved that his background secret has not been revealed.
4. When Armand does see the evidence of his sad past in his baby son, and Desiree asks him if he wants her to leave, he replies, "Yes, go." The narrator explains,
He thought Almighty God had dealt cruelly and unjustly with him; and felt, somehow, that he was paying Him back in kind when he stabbed thus into his wife's soul. Moreover, he no longer loved her, because of the unconscious injury she had brought upon his home and his name.
This passage suggests that Armand has known of his heritage, or, at least, suspected it. Now, with the evidence of his racial background in the appearance of his son--"this unconscious injury"--in order to continue deflect any suspicion of himself, Armand feels he must reject Desiree, whom he blames for this exposure. So, when she turns to him as she is about to depart with her baby,
He did not answer her. That was his last blow at fate.
5. After Desiree's departure, Armand burns anything connected to her or to the baby. At last he burns Desiree's letters. However, pushed to the back of the drawer, there is
the remnant of one back in the drawer from which he took them. But it was not Désirée's; it was part of an old letter from his mother to his father.
The fact that this revealing letter of his mother's race is pushed back behind the letters of Desiree hints further that Armand has known the truth of his heritage.