Preheat oven to 375 degrees. Grease and flour a 13 x 9-inch baking dish and set aside.

In a large bowl, toss together 1 ¼ cups flour, oats, ⅔ cup sugar, baking powder, and butter. Lightly press oat mixture into the bottom of prepared dish. Bake until crust begins to brown around edges, approximately 15 minutes. Let cool for 30 minutes on a wire rack.

Reduce oven temperature to 325 degrees.

In a large bowl, beat cream cheese at medium speed with an electric mixer until creamy. Add remaining 1 cup sugar and remaining 2 tablespoons flour; beat until blended. Add eggs, one at a time, beating just until blended. Add lemon juice and vanilla. Spread blueberries over cooled crust; pour cream cheese mixture over blueberries.

Bake until a knife inserted 1 inch from the edge of baking dish comes out clean and the center feels firm when lightly touched, approximately 40 minutes. Let cool for 2 hours on a wire rack.

Cover and refrigerate until thoroughly chilled, approximately 3 hours. Cut into squares. Garnish with confectioners' sugar just before serving, if desired.