The poem is two uneven stanzas, one that consists of sixteen lines and one of eight lines. It's written in a regular, measured pattern called iambic pentameter. This means each line has five ("penta") metrical feet (two syllables to a foot, so ten syllables in all), which are iambic (one unstressed syllable followed by a stressed syllable). Iambic pentameter is a really common meter in English-language poetry. It sounds like this: ba-DUM ba-DUM ba-DUM ba-DUM ba-DUM. Here's an example of what it looks like in "Blackberry-Picking," with the stressed syllables in bold and italics:
At first, | just one, | a gloss|y pur|ple clot.
Though it's written in regular iambic pentameter, there are a few variations. All the lines are ten syllables, but the iambic pattern bends a bit sometimes. This is all right, though (well within the formal rules). These variations from the iambic foot include:
The lines are also rhymed. Just as an FYI, Heaney is a master at formal verse – poetry that makes use of meter, rhyme, or any of the fixed forms, like the sonnet. This usually seems effortless, though, so the reader is never bugged by forced rhymes or awkward rhythm. He's super smooth.
The rhyme in "Blackberry-Picking" is pretty slick. It's AABBCCDDEEFF and so on and so forth until the end of the poem. So the first line ends with "sun," which rhymes with the end word of the second line, "ripen" (just pronounce it "ripe-un"). The third line ends with "clot" which rhymes with the end word of the fourth line, "knot." The pattern continues throughout the entire poem.
The reason we're not annoyed by the rhyme is because most of the rhymes are half or slant rhymes. Half rhymes are just what they sound like: words with usually a consonant sound in common, but nothing else. They're words that sound an awful lot alike, but not such perfect matches as "cat" and "hat." Heaney has some lovely half rhymes, like "drills" with "full," and "byre" with "fur."