Harvest:
Pick blackberries when the fruits are ripe; ripe berries will turn from bright red to blackish red and from shiny to dull. Ripe fruit will be soft and come away from the plant with little effort. Unripe berries will not ripen off the plant.

For more information on growing Blackberries check-out this great guide from Harvest To Table:
https://harvesttotable.com/how-to-plant-grow-prune-and-harvest-blackberries/