Preheat oven to 350 degrees. Butter a 2-quart shallow baking dish; set aside. In a medium bowl, toss blueberries with 1/2 cup flour, the granulated sugar, and lemon juice. Spread evenly in prepared baking dish, pressing down lightly.
In a small bowl, mix together brown sugar and the remaining 1/2 cup flour; add butter. Using your fingertips or a pastry blender, mix the butter into the dry ingredients until it looks like coarse irregular crumbs.
Spread crumbs over blueberries. Bake for 10 minutes. Reduce the oven temperature to 325 degrees. and bake until the crumbs are browned and the juices have thickened, about one hour. Serve warm.