At first blush, Seamus Heaney's "Blackberry-Picking" seems straightforward. As the title suggests, the poem is a nostalgic remembrance of the subject's youth, which he spent as a boy picking blackberries in the late summer. The poem chronicles the impatience with which he and his friends waited for the fruit to ripen, the greed with which they collected the ripened berries in makeshift containers, and the sadness they felt when much of what they harvested rotted before they could enjoy it.
Delving deeper, there is much more going on with this poem. Some critics have interpreted the poem to be about youth -- about how those who are young take time to "ripen" and to "grow" in experience, but that once they have, that youthful period is fleeting. Others have asserted that the theme is more about the impermanence of all of life. In that interpretation, all things come to an end, so it is best not to try to hold on to an experience (as the boy tries to hoard the berries), but rather to live in, and fully experience, the present moment before it passes.
Whether the theme of passing youth or of general impermanence resonates with you, Heaney uses several poetic devices to get his point across. His use of the metaphor of "summer's blood" for the blackberry juice and the accompanying bloodlust mentioned in the next line (lines 6-7) suggest something primal in the search for the blackberries, something that fulfilled a deep human need. Likewise, Heaney's careful use of action verbs like "scratched" and "bleached" (line 10) communicates the feeling that other elements in nature worked to prevent the boys from accessing the blackberries. The alliteration (repetition of an initial consonant sound) in line 21 serves to draw the reader's attention to the line: "The fruit fermented, the sweet flesh would turn sour." This is important, as it is here that the lovely image of the blackberries changes to one of rotting fruit. In addition, Heaney's juxtaposition of "lovely canfuls" and "rot" (line 23) further drives home the transformation of the berries that has taken place over the course of the poem. Finally, while the poem has followed somewhat of a rhyme scheme, the last two lines not only rhyme, but also have strong-sounding endings. The author no doubt crafted the strong ending purposely to leave this final thought in the mind of the reader: although he always wanted the berries to last longer, in the end, they did not. No amount of his effort could make the berries live longer. By that final line of the poem, we realize that there is far more going on in this poem than a simple berry-picking session.