"Song" is a ballad-style poem, which was first published in Tamerlane and Other Poems in 1827, the speaker tells of a former love he saw from afar on her wedding day. A blush on her cheek, despite all the happiness around her, displays a hidden shame for having lost the speaker's love.
It is believed to reference Poe's lost teenage love Sarah Elmira Royster, who broke off her engagement with Poe presumably due to her father. She instead married the wealthy Alexander Shelton. If this is the case, Poe was taking poetic license: he was not in Richmond at the time of her wedding.