Don't panic, the hyphen in the title means nothing. Well it does let us know (if we didn't already) that our author is British (Irish, to be specific). Though we all speak the same language, there are sometimes little quirks that separate British and American spelling and punctuation. But it's all the same in the end.
This is not a hide-and-seek title. In other words, we don't have to wrack our brains trying to figure out what relation it has to the poem, because right away we see it's indeed about blackberry picking (at least on one level). However, it also shows restraint on Heaney's part. The title doesn't give any hint of what the deeper meaning of the poem is, and instead lets us slowly unveil the possibilities as we read through it.