In one of the bowls, toss the blueberries with the cinnamon and lemon juice. Pour into the prepared baking pan.
In the other bowl, whisk together the flour, ¾ cup sugar, and baking powder. Stir in the milk and melted butter. The batter will be very thick. Spoon over the blueberries.
In the same bowl, stir together the remaining sugar and cornstarch and sprinkle over the batter. Slowly pour the boiling water all over the pan. The water will rise up and pool, but it will combine with the batter as it cooks.
Bake in the preheated oven for 55 to 60 minutes, until the cake tests done and is golden and a little crunchy on top. Serve warm with ice cream.