In a medium saucepan, bring blueberries, sugar and lemon juice to a boil. Lower heat and simmer about 10 minutes, or just until tender.
In a small bowl whisk the cornstarch with 2 tablespoons of water. Add the vanilla then whisk into the blueberries and return to a boil, stirring until thickened another 1 minute or so. Remove from heat and cover with plastic wrap directly on the surface, then refrigerate until cooled, an hour or so.
In large mixing bowl, stir together the oats, flour, sugar, lemon zest, salt and cinnamon. With a pastry blender or 2 dinner knives, cut in the butter until the crumbs are coarse and the size of peas.
Lightly spray the baking pan with cooking spray then press in half of the crumbs. Spread evenly with the blueberry filling, then scatter the remaining oat mixture over top, pressing lightly.
Bake in the centre of a 350°F oven until the crumbs are light golden, about 45 minutes. Cool on a wire rack before cutting into squares.