Preheat oven to 400 degrees F (200 degrees C). Grease a 9x13 inch baking pan.
Start by making the crust. In a medium bowl, stir together the 2 cups flour, 2 tablespoons white sugar, lemon zest and salt. Cut in the 1/2 cup butter until the mixture resembles coarse crumbs. Beat egg and vanilla together; stir into the crumb mixture until a dough forms. Press into the bottom of the prepared pan.
Bake for 12 to 15 minutes in the preheated oven, until golden. Remove from oven and set aside to cool slightly.
Sprinkle blueberries over the crust. Combine the 1/4 cup sugar and nutmeg; sprinkle over the blueberries. Make the topping: In a medium bowl, cream together the 5 tablespoons butter and brown sugar until smooth. Mix in the flour, so that the mixture is crumbly like streusel. Sprinkle over the blueberry layer.
Bake for 20 to 25 minutes in the preheated oven, until browned. Cool and then dust with confectioners' sugar before cutting into bars.