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.