Preheat oven to 375 °F. Line a 13x9-inch baking dish with foil, leaving ends long to use as handles later. Lightly grease foil.
First make the crust layer. In a large bowl, combine white whole wheat flour, 3/4 cup white flour, oats, 2/3 cup sugar and baking powder. Pour in melted butter and cut together with a fork until crumbly and evenly mixed. Transfer this crust mixture to prepared baking dish and gently press crumbs into bottom of pan. Bake at 375°F for 13 to 15 minutes or until lightly browned. Allow cool for 30 minutes in the pan, on a wire rack.
Meanwhile, make filling. In a large bowl, beat cream cheese with an electric mixer until smooth and fluffy. Add 1 cup sugar and 2 Tablespoons white flour and beat until blended. Next, mix in eggs one at a time, until thoroughly combined. Add vanilla, lemon juice and zest, mixing until just combined. Set aside.
Next, scatter blueberries evenly over cooled crust. Pour on cream cheese mixture. Bake at 375°F for about 40 minutes or until a knife inserted an inch from the outer edge of the pan comes out clean. Allow to cool in pan for 2 hours at room temp,then transfer to a refrigerator to chill completely, approximately 3 hours.
When ready to serve, remove chilled bars from pan by lifting up on long ends of foil. Remove the foil, and use a sharp knife to cut into squares. I was serving my bars for dessert, so I cut them into 2 ½-inch squares – a little larger than average. Dust bars with a bit of powdered sugar just before serving if desired.
Keep leftover bars refrigerated in an airtight container, for up to three days.
Notes: Feel free to substitute other fresh (not frozen) berries for blueberries.