A thin layer of frost on your berries can be a sign of freezer burn, which occurs when the berries have been exposed to air. While it doesn't make the berries unsafe to eat, it can lead to changes in texture and flavor. To prevent this, make sure your berries are stored in air-tight containers or freezer bags. If your berries are heavily frosted or have been in the freezer for a long time, it might be best to use them in cooked applications like sauces or jams.