The entirety of "Blackberry-Picking" functions as an extended metaphor, unfolding and developing the poem's central comparison (of blackberry-picking to growing up and losing childhood innocence) across the poem's two stanzas. In the first lines, the berries ripening "For a full week" suggests impatience, much like the impatience of a little kid who wants to grow up and be able to do "adult" things like stay up late. These first five lines speak to childhood innocence, as is reflected by the fact that the newly ripe berry's "flesh was sweet" and unspoiled.
From line 7 onwards, however, the first stanza develops the metaphor of blackberry-picking in new directions, using almost violent language ("briars scratched" and "wet grass bleached") and vivid imagery to mirror the tumultuous process of growing up and transitioning from childhood to adolescence. The blackberries end up "Leaving stains" which can be seen as the marks of tough experiences people increasingly accumulate as they get older. However, people, especially younger people, still strive to collect more experiences, comparable to the children's "lust for / Picking." Younger people in particular are eager and less cautious about entering potentially painful situations, like falling in love and opening oneself up to the possibility of heartache. By the end of the first stanza, the children's hands are "peppered / With thorn pricks" and the experience of blackberry picking itself is concluded, indicating that the children may have reached a new stage of maturity.
The second stanza reflects the aftermath of such an experience, hinting at the way that many life experiences become marred by loss, disappointment, and difficulty. The berries rot (the experience sours) and despair ensues. This stanza points to the difficulty of accepting that hard experiences are simply a part of life and, in particular, of growth. The extended metaphor thus functions to display the entire cycle of a "growing" life experience, from the anticipation of the experience, to the challenges of the experience itself, and finally to the lasting marks these challenges leave behind.
The metaphor is carried through to the very end of the poem and plays one last important role with the final line: "Each year I hoped they'd keep, knew they would not." The speaker is referencing the blackberries rotting and expressing a sad acceptance of the fact that they always do spoil. This can be compared to an adult kind of wisdom, namely the resignation older persons tend to develop regarding life's losses and disappointments. The more difficulties people experience, the poem suggests, the more resigned they become to such challenges. The metaphor reflects this by concluding with resignation and acceptance, sentiments that tend to only develop later in life, once the tough process of growing up is in the past.