Preheat oven to 425°F. Line a 12-cup muffin tin with paper liners and coat them with cooking spray. In a medium bowl, whisk in flours flaxseed, 2½ teaspoons baking powder, ½ teaspoon cinnamon and ¼ teaspoon salt.
In a food processor, combine 6 pitted dates and 1 banana. Process until mostly smooth, about 1 minute, scraping down the sides as needed. Add ⅓ cup oil, ⅓ cup yogurt and 1 teaspoon vanilla. Process again until smooth and creamy, about 1 minute. Add 2 eggs and pulse until just combined, 2 to 3 pulses.
Pour the date mixture into the dry ingredients; gently stir until just a few streaks of flour remain. Fold in 1 cup blueberries just until evenly distributed and no dry flour remains. The batter will be thick. Allow to sit for 5 minutes to hydrate batter.
Divide the batter among the prepared muffin cups (about ¼ cup each). Bake for 5 minutes. Without opening the oven door, reduce the oven temperature to 350°F and continue baking until a wooden pick inserted in the center comes out clean, 12 to 13 minutes more.
Let the muffins cool in the pan on a wire rack for 5 minutes. Remove from the pan and let cool completely on the rack, about 20 minutes.
