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.