Preheat oven to 350°F Line 9×13-inch metal baking pan with foil, leaving a 1-inch overhang on the ends. Spray foil with cooking spray- bottom and sides of the pan.
In a large bowl, whisk together flour, oats, coconut, sugar, salt, and baking powder. Using a pastry cutter (or your fingers), blend the butter completely into the flour mixture. Transfer 2 cups of the crumb mixture to another bowl and reserve for the 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. Use the bottom of a flat, wide glass to tap the mixture and even it out.
Bake the crust 10-12 minutes, or until it starts to form a dry top.
Meanwhile, in a medium bowl, whisk condensed milk, lemon juice & zest, and egg yolk. Let mixture stand for 5 minutes (it will begin to thicken).
Sprinkle blueberries evenly over hot crust and then drop spoonfuls of the lemon mixture over the blueberries. Spread gently w/ a spatula to distribute as evenly as you can. Bake until lemon mixture begins to form a shiny skin- 7 to 8 minutes.
Sprinkle reserved crumble topping over the lemon-blueberry layer, pressing the streusel between your fingers into small lumps as you sprinkle. Bake until filling is bubbly 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. (If you have time to chill the bars, they’re easier to cut cleanly when chilled).