Preheat oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease it lightly.

In a large bowl, whisk whole wheat flour, baking powder, baking soda, salt, and cinnamon until combined.

In another bowl, whisk eggs, unsweetened applesauce, Greek yogurt, vanilla, and mashed banana until smooth.

Pour the wet ingredients into the dry mixture and stir gently until just combined — avoid overmixing.

Fold in blueberries and lemon zest if using, distributing them evenly through the batter.

Add milk a tablespoon at a time if batter seems too thick; it should be scoopable but moist.

Divide the batter evenly among muffin cups, filling each about two-thirds full.

Bake until muffin tops are golden and a toothpick inserted in the center comes out clean, about 18–22 minutes.

Allow muffins to cool in the pan for a few minutes, then transfer to a wire rack to cool completely.