Preheat oven to 350°F. Lightly coat a 9-inch square baking pan with cooking spray. Place ½ cup blueberries in a medium microwave-safe bowl; lightly mash with the back of a fork. Microwave on High for 30 seconds. Stir, lightly mashing the blueberries again, then microwave on High for 30 seconds more. Stir in 2 teaspoons chia seeds and 1½ teaspoons honey. Let stand until the liquid is absorbed and the mixture has a jammy texture, at least 5 minutes.

Meanwhile, whisk 1¼ cups oats, ½ cup all-purpose flour, ½ cup whole-wheat flour, 1½ teaspoons baking powder, 1 teaspoon cinnamon and ½ teaspoon salt together in a small bowl. In a large bowl, whisk mashed banana, 4 tablespoons peanut butter, ¼ cup milk, ¼ cup applesauce, 1 egg, 1½ teaspoons vanilla and the remaining 2½ tablespoons honey together until combined. Fold the oat mixture into the banana mixture until just combined and no dry streaks remain.

Transfer half of the oat mixture to the prepared baking pan, spreading it into an even layer. Dollop evenly with half of the blueberry mixture (about 2½ tablespoons); top with the remaining oat mixture, spreading into an even layer. Dollop the remaining 2 tablespoons peanut butter and the remaining blueberry mixture evenly on top. Using a wooden pick or a paring knife, drag the peanut butter and blueberry dollops through the oat mixture, creating a marbled pattern.

Bake until the edges begin pulling away from the sides of the pan, about 30 minutes. Let stand in the pan on a wire rack until cool enough to handle, about 10 minutes. Slice into 9 squares and serve warm.