Make the cream puffs: Preheat oven to 400 degrees. Combine water, butter, granulated sugar, and salt in a medium saucepan. Bring to a boil, and remove from heat. Stir in flour until combined, and return to medium heat. (This mixture is called a panade.)
Dry the panade by stirring constantly for 4 minutes. It is ready when it pulls away from the sides and a film forms on the bottom of the pan.
Transfer to the bowl of an electric mixer fitted with the paddle attachment, and mix on low until slightly cooled, about 2 minutes. On medium speed, add 4 eggs, one at a time, incorporating each completely before adding the next. Lightly beat fifth egg, and add a little at a time until batter is smooth and shiny. Test by touching it with your finger and lifting to form a string. If string does not form, the batter needs more egg. If you've added all the eggs and the batter still doesn't form a string, add water, 1 teaspoon at a time, until it does.
Using a pastry bag fitted with a plain 1/2-inch round tip, pipe ten 2 1/2-inch rounds of batter onto parchment-lined baking sheets. Brush with egg wash, and sprinkle with sanding sugar. Bake for 15 minutes. Reduce temperature to 350 degrees, and bake until puffed and golden and centers are dry, about 35 minutes more. Let cool on a wire rack.
Make the blueberry sauce: Bring 1 cup blueberries, the water, granulated sugar, lemon zest, and salt to a boil in a small saucepan. Reduce heat, and simmer until most of the berries have burst, about 5 minutes. Remove from heat. Add remaining cup blueberries, and crush lightly.
Make the lemon mousse: Lightly whisk lemon curd, and fold in whipped cream using a rubber spatula. Halve each cream puff, and fill bottoms with lemon-mousse. Spoon on blueberry sauce, replace tops, and serve.