But I know, too, That the blackbird is involved In what I know. (Section VIII)
This guy has birds on the brain. Everything he looks at is colored in some way by encounters with the blackbird. We wouldn't be surprised to open up his brain and find a blackbird pulling the strings like a puppet-master.
Once, a fear pierced him, In that he mistook The shadow of his equipage For blackbirds. (Section XI)
Stevens uses deliberately old-fashioned words and images in this section, adding a new layer of reality to the poem: historical time-travel! This is another section that needs to be felt, not interpreted.