Preheat the oven to 350°F (175°C). Grease a 9x9 inch baking pan.

In a mixing bowl, combine flour, oats, brown sugar, granulated sugar, melted butter, vanilla extract, baking powder, and salt. Mix until crumbly.

Press half of the mixture into the bottom of the prepared baking pan to form the base.

In another bowl, combine blueberries, lemon juice, cornstarch, and granulated sugar. Toss gently to coat the blueberries.

Spread the blueberry mixture evenly over the base in the baking pan.

Crumble the remaining base mixture over the blueberry layer.

Bake in the preheated oven for 25-30 minutes or until the top is golden brown.

Allow to cool completely before cutting into bars.