Pre-heat oven to 350 F
In a large saucepan, combine cornstarch and sugar and whisk until combined. Add in water and whisk until smooth. Add blueberries; bring to a boil slowly. Stir constantly until filling thickens. Remove from heat and set aside at room temperature.
Line 8 inch pan with parchment paper.
Meanwhile, in a large bowl stir together oats, flour, sugar and salt. Drizzle with melted butter and stir until well mixed.
Spread half the mixture into lined pan. Cover with blueberries filling and spread the remaining oatmeal mixture on top, packing down lightly with a fork.
Bake in 350 F oven for 40 minutes or until golden. Let cool before cutting into squares.