In general, any food with a skin will have a harder time freeze drying. Blueberries are especially difficult to freeze dry. If possible, purée such foods and freeze dry the purée. If that is not possible cut the berries into halves or quarters. Additionally, using a tooth pick or other small sharp object to puncture holes in each of the berries can help. This technique can be used for blueberries, cherries, grapes, huckleberries, etc.