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). Whisk in the eggs, vanilla, and oil. (I like to use a 4 cup measuring cup and just whisk it all in there, but you can use a bowl if your measuring cup isn’t big enough to hold all the liquid.)
Pour milk mixture 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.
Eat warm, or slice and store in an airtight container in the refrigerator. Or place in ziploc baggies and freeze for up to 1 month. Great for reheating in the microwave for fast breakfasts!