Fresh blackberries are best used immediately, but they may be refrigerated in a non-airtight container lined with paper towel, preferably in a single layer, for one to two days. They should be washed only just prior to use. To prevent blackberries sticking together when freezing them, place them on a cookie sheet so that they are not touching each other, and then in the freezer; once frozen, put the berries in a sealable plastic bag.