"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.