Fresh is always better! Always use fresh berries, blackberries in this muffin recipe. But that being said, if blackberries aren't in season, frozen can be used. Don't defrost them, just add them frozen to the batter. Frozen berries release more liquid when they melt, so I would recommend mixing them with a little more flour before mixing with the batter. Mixing frozen berries into your muffin batter can also result in a littler longer baking time. Check the berry muffins toothpick inserted into the center of a muffin comes out clean.