If it's not blueberry season and you can't get fresh fruit (or because they're just overpriced outside of the season), you can use store-bough frozen berries. You just want to make sure that the frozen berries are thawed before using them. Using frozen berries that come from your store's freezer section can retain water and throw off your jam's consistency and taste. If you pick fresh blueberries from your own bush, like us, but don't have a cup yet, you can put those in a glass container and flash freeze them (just put that in the freezer). When you have enough berries for jam, take those out and gently roll them in your hands to warm them before using them. You don't need those to thaw completely.