In "Blackberry-Picking," Heaney tackles an age-old poetic idea: we're all going to die eventually. But he does it in a pretty relatable way. By showing us the rotting berries (and the desire to keep them from rotting), he reminds us of how we cling to life in spite of our knowledge of death. It's pretty moving. We all want to keep what won't stay; the fact that these blackberries are impermanent or temporary is what makes them so precious. So the berries, because they're perishable, are something to treasure in the speaker's eyes.
The desire to collect all of the fresh berries is an attempt to take in the most of life and, in turn, is an effort to keep death away.
"Blackberry-Picking" really does show us the downside of expectation: disappointment. In the space of the poem, Heaney shows what it's like to wait for something, to grow excited about it, to work for it, and then to watch it all fall away. What a bummer!
Considering the ending of the poem, Heaney's saying that the only reason we're ever disappointed is because we have our hopes set too high. Expectation only creates a feeling of loss and letdown.
The boy in "Blackberry-Picking" is just about that age when sex (or at least ideas about sex) is starting to become part of life. The desire to gather and eat the berries represents the desire to, well, you know, do something like that but with women in place of berries. Ever heard of the forbidden fruit?
The blackberries in the poem could have been replaced by any fruit and had the same effect.
Well, the kid can't eat just one! He wants tons and tons of berries, a whole secret stash just for himself. He's obsessed with searching for them. He carries berries in anything that will hold them. He stores them in a bath (now that's a lot of berries). "Blackberry-Picking" shows us the determined energy behind greed and its negative consequences.
Though Heaney shows the boys' greed, the poem's tone isn't a punishing one. He sees greed as something inherent to their age.
Heaney's main message in "Blackberry-Picking" is, "nothing's permanent, and we never get used to it," and that's what's important to remember. But Jesus Christ also plays a big role in this poem, complicating that message a little. If you think about it, it's the Christian belief to be all right with the death of everything on earth because there is Heaven to look forward to. From the Eucharist to the crucifixion, Jesus really dominates the symbolism. But keep in mind that although religion and religious imagery are big in this poem, that doesn't mean the poem is really "about" that. We wouldn't call this a Christian poem. It's much more universal than that. The Christianity just seems to be there because it was a big part of the speaker's youth.
The poem is about punishment issued by the Christian God for the speaker's sins of lust and greed.