Preheat your oven to 350°F and prepare a muffin tin with paper liners or a light coating of oil.
In a blender or food processor , combine the bananas, Greek yogurt, eggs, oats, maple syrup, baking powder, baking soda, salt, and vanilla protein powder (if using). Blend until the mixture is smooth and well combined.
Gently stir in the blackberries by hand, being careful not to crush them, to keep some whole berries in each muffin.
Pour the batter into the prepared muffin tin, filling each cup about 3/4 full. Bake for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
Let the muffins cool in the tin for a few minutes before transferring them to a wire rack to cool completely. Enjoy warm or at room temperature!