To make the crust: Preheat oven to 375 degrees F.
Line a buttered 8-inch square glass baking dish with 2 (18 by 6-inch) sheets of foil, overlapping them in opposite directions so that there is overhang on all 4 sides.
Pulse together flour, cornmeal, confectioners' sugar, salt, and butter in a food processor until mixture resembles coarse meal. Press onto bottom of baking dish and 1-inch up sides. Bake in middle of oven until golden brown, about 20 minutes.
To make the filling while crust bakes. Whisk together eggs, sugar, flour, and zests. Whisk in juices, milk, and a pinch of salt. Toss blueberries with jam in another bowl.
Whisk egg mixture and immediately pour into hot crust, then bake until just set, about 17 minutes. Gently spoon berries evenly over top and bake 2 minutes more. Transfer baking dish to a rack and cool.
Chill, covered, overnight (8 hours). Use foil to lift dessert out of dish, then cut into squares.
Recipe courtesy of Gourmet Magazine