Blackberries are a source of food and energy on Tinkatolli. They were the 4th type of food to release, after blueberries, strawberries, and lingonberries. They first appeared in October 2011, in the first Halloween party.
Like the Strawberries and Lingonberries, they are only available in Canopia. They are party-only foods. These go great at halloween because they are somewhat spooky.
After the halloween party, they are probably going to go, and thereafter be available only in October.