Among twenty snowy mountains, The only moving thing Was the eye of the blackbird. (Section I)
You could imagine this opening as a scene in a film that begins in a huge white landscape and slowly zooms in on a tiny black speck until the blackbird comes into view, and then keeps zooming until the whole screen is filled with the bird's eye. See! It twitched!
The blackbird whirled in the autumn winds. It was a small part of the pantomime. (Section III)
Little did we humans know, but there's the equivalent of a Broadway show going on at every moment out in the nature. The random activities of nature are compared to bit roles in a theatrical performance. The speaker views nature through the eyes of culture.
Do you not see how the blackbird Walks around the feet Of the women about you? (Section VII)
This section implies that a blackbird is just as good as some hoity-toity "golden bird." When you think about it, the value of gold is really cultural: we think of gold as beautiful because it's rare and exceptional. We often neglect the commonplace beauties that can be found everywhere.
I know noble accents And lucid, inescapable rhythms; But I know, too, That the blackbird is involved In what I know. (Section VIII)
"Nobility" is a "civilized" characteristic. The highest activities of human culture, such as poetry and music, have traditionally been considered "noble." The blackbird's involvement in nobility and so-called "high" culture is meant to surprise us.
At the sight of blackbirds Flying in a green light, Even the bawds of euphony Would cry out sharply. (Section X)
The "bawds of euphony" don't pay much attention to nature because it's too complex, chaotic, and ambiguous. But even they can't resist the arresting contrast between dark black and bright green.