I really recommend not doing this poem tomorrow its tough poem that is quite hard to answer, but if i was really stuck and had to answer with the poem i would talk about the imagery in the poem and how he compares blackberries to "summers blood" "a glossy purple clot" and "its flesh was sweet".
I would talk about the senses in the poem:
Taste; "sweet" "sour"
Smell; "the juice was stinking too"
Touch; "our hands were peppered with thorn pricks, our palms sticky as Bluebeard's"
Sight: "big dark blobs" "a glossy purple clot"
There's also a change in tone between the first and second half of the poem, he is excited and thrilled that he and his friends have gathered the blackberries and is excited for the summer ahead, but by the second half when a "rat grey fungus" grows on the blackberries in the tub the poet is torn and hopes that each summer they would keep but of course would not
Sorry if this is really bad the notes my teacher gave out were useless, good luck tomorrow hopefully this helped a bit