Position rack in center of oven and preheat oven to 350 degrees.
Line a 9x13" metal baking pan with foil, leaving a 1" overhang on the ends. LIghtly butter the botton and sides of the foil.
In a large bowl, combine flour, oats, sugar, salt and baking powder. Using your fingers, blend 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. Tamp it down with the bottom of a measuring cup to even it out. Bake crust until starts to form a dry top, 10-12 minutes.
Meanwhile, in a medium bowl, whisk the condensed milk, lemon juic, lemon zest, and egg yolk. Let this mixture stand for 5 minutes; it will beginto thicken.
Sprinkle the blueberries evenly over th hot crust and 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 blueberries. It's fine if lemon mixture isn't perfectly even. Bake until the lemon mixture just begins to form a shiny skin, 7-8 minutes.
Sprinkle the reserved topping over the lemon-blueberry layer, pressing the streusel between your fingers into small lumps as you sprinkle. Bake until filling is bubbling at the edges and top is brown, 25-30 minutes.
Let the 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 rack to cool completely. Remove the foil and cut into 24 bars when cool. The bars may be stored at room temperature for a few hours but otherwise should be kept in the refrigerator.