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.