"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!
Questions About Disappointment
At what point in the poem did you start to realize this might not turn out well for the speaker? When you reread the poem, do you notice it sooner?
What connection can you make between the time of year and the boy's disappointment?
If he's disappointed year after year, why doesn't he ever learn?
Chew on This
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.