Position a rack in the center of the oven. Preheat oven to 350 degrees. Line 9 x 13 inch baking pan with foil, leaving a 1-inch overhang on the ends. Lightly butter sides and bottom of the foil.
In large bowl, combine the flour, oats, sugar, salt, and baking powder. Using your fingers, blend the butter completely into the flour mixture. Transfer 2 cups of crumb mixture to another bowl and reserve for topping.
Blend the egg white into the remaining crumbs and then press the mixture into the bottom of the pan to form a level crust. Bake crust until it starts to form a dry top, 10 to 12 minutes.
Meanwhile, in a medium bowl, whisk the condensed milk, lemon juice, lemon zest and egg yolk. Let this mixture stand 5 minutes; it will begin to thicken.
Sprinkle blueberries evenly over the hot crust and then drop spoonfuls of the lemon mixture over the blueberries. Spread gently with a spatula to distribute a little more evenly, but take care not to crush the berries. Its fine if the lemon mixture isn't perfectly even.
Bake just until the lemon mixture begins to form a shiny skin, 7 to 8 minutes. Sprinkle reserved topping over the lemon blueberry layer, pressing streusel between your fingers into small clumps as you sprinkle. Bake until filling is bubbling at the edges and the topping is brown, 25 to 30 minutes.
Let bars cool in the pan on a rack until just warm, about an hour. Carefully lift them out of the pan using the foil overhang and transfer to a wire rack to cool completely. Remove foil and cut into 24 bars when cool. May be stored at room temperature for a few hours, but otherwise should be kept in the refrigerator.