Blackberries will not ripen off the vine. Only pick black, plump berries that easily pull from the plant. Do not wash blackberries until you are ready to use them. They will stay fresh in the fridge for about a week. If you want to save them for a rainy day, freeze them. Arrange the berries in a single layer on a cookie sheet and keep in the freezer overnight. Once frozen transfer the berries to an airtight container.