The speaker compares two ways of knowing the same thing. Someone else, "he," can name each bird and say which song it sings. The speaker knows only the songs, never the birds behind them. Because of that, the songs blur together, as if every bird the speaker ever met were singing at once, "together in harmony." The speaker believed that meeting every bird meant knowing every song, and that each song was the same every time it was heard.
The last stanza shifts into a quieter, humbler tone. "I'm coming to learn" suggests a realisation still in progress: there will always be more songs and more birds than the speaker can learn. The title, "Blind," seems to point to this limit, since hearing without seeing leaves something missing. The mood is reflective and a little wistful, but not bitter. The speaker accepts that knowing is never finished, however hard they try.