Fresh, in-season blackberries are recommended for this recipe because they will give you the best flavour, texture, AND result in better muffins, overall. However, if it’s the middle of winter or another time when you can’t find them fresh, frozen can be used as a substitute. To use frozen berries, thaw them completely on paper towels, pat away all extra moisture (this is important because any extra moisture can alter the recipe), and then chop them in half, width-wise if needed. Don't mix the batter too much when you add the berries or the batter will turn purple.