Butter a casserole dish (about 11 x 7). Cut french baguette into cubes. Place half the cubes in an even layer across the bottom of the casserole.
With a stand or hand mixer, whip together the cream cheese, powdered sugar, vanilla and lemon zest. Carefully spread the cream cheese mixture on top of the bottom layer of bread cubes... using a spatula and a clean fingertip.
Layer the blueberries evenly atop the cream cheese layer. Place the other half of the bread cubes atop the blueberries.
Whisk together the eggs, milk and granulated sugar. Pour evenly over the top of the bread. Push down the top of the bread so it all gets evenly coated with the egg mixture.
Seal the dish with foil and refrigerate overnight.
The next morning, remove the dish from the refrigerator before you preheat your oven to 350 degrees.
Bake for 30 minutes covered. Remove foil and bake for 30 minutes more. Serve with blueberry sauce.