Preheat oven to 350ºF. In a small bowl, soak dates in apple juice for 10-15 minutes. Lay a piece of parchment paper in a 9 x 9-inch baking pan, making sure the sides are covered. Cut slits in the corners of the paper so that the paper overlaps and is flat.
In a medium bowl, combine flour, oats, cinnamon, and nutmeg. Mix and set aside. Place bananas and vanilla extract into a blender. Remove dates from apple juice and set aside. Add the strained juice to the blender and blend until creamy. Add dates to the blender and pulse a few times so the dates are in small pieces.
Pour the banana mixture into the dry ingredient bowl. Mix until well combined. Stir in the blueberries and walnuts. Using a spatula, pour the batter into baking pan. Bake at 350ºF for 30 minutes, or until a toothpick inserted into the center comes out clean. Cool at room temperature for 5 to 10 minutes before cutting and serving.