Preheat oven to 375 degrees F (190 degrees C); lightly grease and flour a 9x13 inch baking dish.
In a large bowl, cream together 1/4 cup butter and 3/4 cup sugar. Beat in egg. Beat in milk. Combine 2 cups flour, baking powder, and salt, and stir into butter mixture. Finally, fold in blueberries and spread mixture into prepared baking dish.
In a small bowl combine 1/4 cup butter, 1/2 cup sugar, cinnamon and 1/3 cup flour. Mix together until mixture resembles coarse crumbs, then sprinkle over blueberry mixture.
Bake 45 to 50 minutes, until topping is golden.