Make the filling: In a bowl, toss the blueberries with the lemon juice, sugar and cornstarch and let stand for 10 minutes.
Meanwhile, Make the topping: Preheat the oven to 375°. Spread the pound cake cubes on a baking sheet and toast in the oven for 10 minutes. Let cool, then toss the cubes with the butter, sugar and lemon zest.
Spread the filling in a 9-by-13-inch baking dish. Arrange the pound cake cubes on top. Cover the dish lightly with foil and bake in the center of the oven for 30 minutes, until the fruit is beginning to bubble. Remove the foil and bake for 20 minutes longer, until the fruit is bubbling and the topping is golden and crisp. Let cool for 1 hour before serving.