Heat oven to 350°F. Line an 8-inch square pan with parchment, leaving overhang.
Whisk flour, oats, brown sugar, 1/4 cup sugar, baking powder, and salt in a large bowl.
Stir in melted butter and vanilla until the mixture forms moist crumbles.
Press about two-thirds of the crumble firmly into the prepared pan to make an even base.
Toss blueberries with remaining 1/3 cup sugar, cornstarch, lemon juice, and zest until coated.
Scatter the blueberry mixture evenly over the crust, including any juices.
Crumble the remaining topping over the berries, leaving some blueberries peeking through.
Bake 33–38 minutes, until the top is golden and the filling bubbles at the edges.
Cool completely in the pan, then lift out and slice into bars.