Adjust an oven rack to the middle position. Preheat the oven to 350F. Grease a 9 x 13-inch baking pan and line it with parchment sling.
In the bowl of a stand mixer fitted with a paddle, mix the flour, oats, sugar, baking powder, salt, and cinnamon on low to combine. Add the butter and mix on medium until the mixture resembles coarse sand.
Press half the flour-oat mixture into the bottom of the prepared pan. Bake 10 minutes. Prepare the berry filling while the crust is baking (see below). Remove the pan from the oven, spread the berry mixture over the crust, and sprinkle the remaining crumble mixture evenly over the top. Bake 20 to 25 minutes, until the crumbly top is light golden brown.
Transfer to a wire rack and let cool. Place the pan in the fridge and let chill 4 to 6 hours. Slice bars and serve. The crumble bars can be served cold or at room temperature, but keep best in the fridge.
For the blueberry filling
In a large bowl, mix the blueberries, apple, and lemon juice. In a small bowl, combine the sugar, cornstarch, and salt. pour the sugar mixture over the berries and stir gently with a spatula to evenly combine.