Preheat your oven to 350°F and line a 12-cup muffin tin with paper liners or lightly grease with oil.
Peel the bananas and add them to a large bowl, mash the bananas until smooth with minimal clumps. Add in the eggs, oil, maple syrup, and vanilla. Whisk until well combined.
In a separate bowl, whisk together the flour, baking powder, salt, oats, ground flaxseed, and hemp seeds.
Using a large spatula or wooden spoon, gently fold the dry ingredients into the wet mixture until just combined. Be careful not to overmix!
Carefully fold in the blackberries. If your berries are extra large, you can slice them in half for better distribution.
Scoop the batter evenly into the muffin cups (fill them to the top!) and sprinkle each with a little granulated sugar.
Bake for about 20-22 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes before transferring to a wire rack.