What does the letter which Armand's mother wrote to his father symbolise in "Desiree's Baby"?

The letter at the end of this excellent story is of course
a wonderful yet devastating example of situational irony, as we realise that the
dark-skinned gene came not from Desiree, as both we and Armand had assumed, but from
himself, and thus he realises that it was his false accusation and terrible behaviour
against his wife that led to her death and the death of his child. Note what the letter
contains:

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"But, above, all," she wrote, "night and day, I
thank the good God for having so arranged our lives that our dear Armand will never know
that his mother, who adores him, belongs to the race that is cursed with the brand of
slavery."

The contents of the
letter show what a shame it was considered to be at the time of the story to have any
"black blood" in your system. Armand's mother thanks God that her son can remain
ignorant of this fact because of the way it would have affected his standing in society,
yet ironically his mother's success in this respect gives rise to Armand's greatest
tragedy, as he loses his loving wife and child. As such, we could argue that the letter
symbolises mistaken good intentions: Armand's mother saw it as a good thing that Armand
remained ignorant of his genetic background, but the tragedy of the story clearly shows
that she was wrong. Equally, we could also argue that the letter shows the danger of
jumping to conclusions: the orphaned Desiree with no known relations seems to be the
easy person to blame for the dark skin of her child, whereas in fact it was Armand who
was responsible. Appearances may not always be what they seem.