the blackberries would ripen. At first just one, a glossy purple clot (lines 3-4)
Where there is a beginning, there will be an end. The poem opens with the start of the berry's life, giving us our first hint that the berry will later have a death. And not to get too graphic here, but think of a newborn baby covered with afterbirth. Resemble anything in the berry's description here?
summer's blood was in it (line 6)
If we didn't want to compare the berry to a human thing at the beginning, it's sure getting harder now. Instead of juice, we have blood, which makes it seem more human. And we all know that part of being human is our mortality.
big dark blobs burned Like a plate of eyes. (lines 14-15)
The comparison of the berries to humans will not be ignored! Heaney is trying to humanize the berries as much as possible to make the message of mortality that much more powerful. Because, honestly, who cares if a few berries rot? But relating it to our own fate makes it much more important to us.
the sweet flesh would turn sour. (line 21)
He may as well be talking about his aging lover. Sweet flesh turning sour, or young flesh getting wrinkly, is the reason so many cosmetic companies are in business! Humans are constantly trying to preserve themselves and slow the effects of aging. But we all know there's no stopping it.
Each year I hoped they'd keep, knew they would not. (line 24)
Bummer. This is the age-old desire to stay young and alive. This is why so many fairy tales and myths have some version of the quest for the fountain of youth. Death is not the easiest thing for humans to reconcile. Regardless of how often we're told we're going to die, we try best we can to stretch out life.