It helps to think of "Thirteen Ways of Looking at a Blackbird" as both a longer poem with thirteen sections and as a sequence of thirteen shorter poems. The sections belong together, but to some extent, they work individually. Specifically, each section bears some resemblance to the haiku, a very precise Japanese form that is divided into three sections. In English, conventional haikus are written in three lines with 5 syllables, 7 syllables, and 5 syllables. Obviously, Stevens has no interest in copying the exact form of Japanese haiku, or any other kind of poem, but he captures the atmosphere of a haiku. The haiku often concerns the seasons, as do several of the sections in "Thirteen Ways of Looking at a Blackbird." For example, "snowy" in the first section suggests winter, and the third section specifies "autumn winds." Haikus also frequently contrast intense images of nature, like the blackbirds flying in green light in section X.
The poem has no regular meter, though elsewhere in his work Stevens used traditional meters like iambic pentameter, most notably in his famous, "Sunday Morning." Here he employs lots and lots of enjambment, where one line carries over into the next without a pause. But you probably add a slight pause anyway after the enjambed lines, giving the poem the peculiar formal quality that we often associate with Eastern wisdom:
It marked the edge
Of one of many circles.