Position a rack in the center of the oven and preheat it to 350 degrees.
Line a 9 x 13 inch baking pan with foil, leaving a 1-inch overhang on the ends. Lightly butter the sides and bottom of the foil.
In a large bowl, combine the 3 cups all-purpose flour, 1 1/2 cups old-fashioned rolled oats, 1 1/3 cups light brown sugar, 1 teaspoon salt, and 1 teaspoon baking powder. Stir these dry ingredients together until well blended.
Using your fingers, blend the 1 cup unsalted butter completely into the flour mixture until it resembles wet sand.
Transfer 2 cups of this crumb mixture to another bowl and reserve it for topping.
In the bowl with the remaining crumbs, blend in the 1 large egg white and press into the bottom of the prepared pan.
Bake the crust until it starts to form a dry top, about 10 to 12 minutes.
In a medium bowl, whisk together the 14 oz sweetened condensed milk, 1/2 cup fresh lemon juice, 2 teaspoons lemon zest, and the 1 large egg yolk. Let sit for about 5 minutes.
Once your crust is ready, sprinkle the 2 1/2 cups blueberries evenly over the hot crust.
Drop spoonfuls of the lemon mixture over the blueberries and spread gently with a spatula.
Bake just until the lemon mixture begins to form a shiny skin, about 7 to 8 minutes.
Sprinkle the reserved topping over the lemon blueberry layer, pressing it between your fingers into small clumps.
Bake until the filling is bubbling at the edges and the topping is brown, about 25 to 30 minutes.
Let the bars cool in the pan on a rack until just warm, about an hour.
Carefully lift them out using the foil overhang and transfer to a wire rack to cool completely.
Once cooled, remove the foil and cut into 24 bars.
