A wedding is supposed to be the one occasion when everyone looks at the bride and sees the same thing: joy. Poe's "Song" is spoken by someone who looks and sees something else. The speaker watches a woman on her bridal day, notices a deep blush rising in her face, and cannot stop turning it over. We never find out what the blush meant. We learn only what it did to the man watching.
The speaker hedges at every turn. The light in her eye is "Whatever it might be," and the parentheses around the phrase work like a lowered voice, a refusal to name what he half sees. The blush "perhaps, was maiden shame," and "As such it well may pass." The phrase "may pass" is grudging. It is the explanation that will do for polite company, the one that lets the guests move on. Judging by the wording, he does not quite believe it. The poem never says what he suspects instead. He may hope the blush was for him, fear it was regret, or simply be unable to accept that it was innocent. That gap keeps the poem alive.
The imagery is built from fire. The blush is "burning," the eye has a "kindling light," and the "glow" of the blush "hath raised a fiercer flame" in the speaker's breast. Heat passes from her to him, though nothing suggests she meant it to. The setting around her is soft and full: "happiness around thee lay, / The world all love before thee." Against that abundance, his "aching sight" can find nothing else on earth worth calling Loveliness. She is surrounded by love while he stands outside it, burning quietly.
The form suits this. "Song" is four quatrains rhymed ABAB, with longer lines alternating with shorter, indented ones. It sounds like something that could be sung, and that makes the content more painful. The rhymes are plain (day/lay, light/sight, shame/flame), and the one that matters most comes late: "pass" is answered by "alas!" It is the only exclamation in the poem, and the speaker's composure finally slips there.
The grammar then plays a small trick. In the first two stanzas the speaker says "I." In the third he becomes "him," as if saying "me" were too exposed: "In the breast of him, alas! // Who saw thee on that bridal day." The sentence spills over the stanza break, and only the repeated phrase tells us that "him" is the "I" of the opening line. He has been talking about himself at arm's length.
The last stanza returns almost word for word to the first, but the changes are telling. "A burning blush came o'er thee" becomes "that deep blush _would_ come o'er thee." The italicised "would" is the poem's strangest moment. It suggests insistence, even faint resentment, as though the blush came in spite of everything, and as though he has replayed the scene so often that it now seems willed or inevitable. "Burning" has given way to "deep," so the memory has sunk in and settled. Even the punctuation changes. The first stanza ends on a colon that leads on to more detail. The last ends on a full stop, and there is nothing left to add. The speaker has gone around in a circle and ended where he began.
Poe is best known for "The Raven" and his Gothic tales, where the dread is loud and the settings are dramatic. This poem is slight beside those, but it shows his gift for compression. It has one scene, one image, a few words of doubt, and a refrain that tightens like a knot. The suffering is private and the evidence is thin, and the poem lets us feel how an uncertain moment can take over a mind.