Blackcurrants are a Quest Item required for The River Rescue Quest.
They take 1 day to unlock, and only become available after Sweet Potato has been unlocked.
Farming Blackcurrants takes 8 hours, and yields 25 berries.
20 berries are required to make the Stone Paints.
Although Sweet Potato must be unlocked before the Blackcurrants, it isn't actually used for the Quest at all. the River Rescue is the only task observed thus far where this occurs.