Also called a bramble because it grows on thorny bushes (brambles), the blackberry is the largest of the wild berries. Purplish-black in color, it ranges from 1/2 to 1 inch long when mature. Blackberries are widely cultivated in the United States and are available, depending on the region, from May through August. Look for plump, deep-colored berries sans hull. If the hulls are still attached, the berries are immature and were picked too early; the flavor will be tart. Fresh blackberries are best used immediately but they may be refrigerated, lightly covered and preferably in a single layer, for 1 to 2 days. They are wonderful both for cooking and for out-of-hand eating. In Britain, blackberries and apples are a traditional duo for pies.