Combine the flour, oats, sugar, brown sugar, baking powder, and salt. Add the butter and stir until thoroughly combined.

Press the crust mixture into the bottom of the prepared pan.

Bake 15 to 20 minutes, or until the edges begin to brown.

Place the pan on a wire rack and allow the crust to cool to room temperature.

Using an electric mixer on medium speed, beat the cream cheese until creamy. Add the sugar and flour, and continue mixing until combined.

Add the eggs, one at a time, mixing just until combined after each addition. Mix in the lemon juice and vanilla.

Scatter the blueberries over the cooled crust. Pour the filling mixture over the blueberries.

Bake 40 to 45 minutes, or until a knife inserted about an inch from the edge comes out clean and the center is firm to the touch.

Cool the bars to room temperature. Then, cover and refrigerate at least 3 hours.

Use a sharp knife to cut into bars. If desired, sprinkle with confectioners' sugar or top with sweetened whipped cream just before serving.