Preheat the oven to 350 degrees F.
For the topping: Put all topping ingredients in a bowl and rub together with your hands until mixture sticks together in small clumps.
Pour the blueberries into a 9 by 13-inch baking dish. Squeeze the lemon juice over the berries, sprinkle the sugar and flour over them and toss until the berries are coated with sugar.
Scatter the topping over the berries in an even layer. Bake until the topping is golden brown and the berry juice is bubbling up through the topping, about 45 to 50 minutes.