Preheat the oven to 350 degrees F (175 degrees C). Lightly grease an 11x7 inch baking dish.
Spread the blueberries out to cover the entire bottom of the baking dish. Squeeze the juice from the lemon over them. In a medium bowl, stir together the butter and 1 1/8 cups sugar until smooth. Stir in flour and baking powder alternately with the milk until smooth. Batter will be thin. Spoon over the berries, and spread evenly.
In a small bowl, stir together 1 1/2 cups of sugar, salt, and cornstarch. Sprinkle over the top of the batter. Dust with a pinch of cinnamon, then pour the boiling water over the entire dish.
Bake for 45 minutes in the preheated oven, or until golden brown.