The blackbird whirled in the autumn winds. (Section III)
This is the only point in the poem that we're sure what season it is. But we think it's reasonable to argue that when Stevens uses the word "snowy," he knows that it signals winter. The poem seems to move backward into autumn.
The blackbird whistling Or just after. (Section VI)
The innuendo of the blackbird's whistling depends on catching the moment "just" after the sound dies away, when the impression of the sound remains fresh in the listener's mind. Otherwise, you're just hearing complete silence.
Would cry out sharply. (Section X)
Stevens uses the conditional tense: if this happens, then this "would" happen. If they saw blackbirds in green light, they "would cry out sharply." It's a small detail, but this tense suddenly shifts the poem into the future and the realm of possibility.