Preheat oven to 350°F. Spray a 9x13” baking dish with cooking spray.
Whisk flour, sugar, baking powder, and salt in a large bowl. Set aside.
Place a banana in a large measuring cup and mash it with a fork. Add milk, a little bit at a time, stirring it into the banana, until the level in the measuring cup is 1 2/3 cup total (milk and banana). You will need approximately 1 ⅓ cups milk, but possibly a bit more or a bit less.
Whisk the eggs, vanilla, and oil into the milk mixture.
Pour wet ingredients into the dry ingredients and whisk until smooth. Stir in blueberries and pour into prepared pan.
Bake for 22-25 minutes, until it starts to get slightly golden around the edges and is no longer wet-looking in the center. It won’t get very brown on top. Cool slightly before serving with syrup.