When the blackbird flew out of sight, It marked the edge Of one of many circles. (Section IX)
Again, the speaker's imagination creates the reality surrounding the blackbird. He's not just amazed by the bird; he's amazed by the circles.
Even the bawds of euphony Would cry out sharply. (Section X)
It's hard to rouse those "bawds of euphony," who usually only get excited by very harmonious sounds and images, but the blackbird has them crying out in amazement.