Dark Purple Berries are a rare fruit that grow only on Pessim's Island.
They grow on a bush and are about the size of a gooseberry, and are of a dark purple color. Eating a dark purple berry is the only remedy for the effect of the Lavender Berries, which cause one to shrink to the size of an insect. The Dark Purple Berries will restore one thus shrunk to normal size.
Pessim discovered the unusual properties of these berries after his neighbors deserted him on the island.
When it was time to leave the island, Trot and Cap'n Bill ate of the Lavender Berries so they could be carried away by the flying Ork, bringing Dark Purple Berries with them so they could be restored. (The Scarecrow of Oz)