Beat the softened cream cheese, powdered sugar, 2 tablespoons milk, and 1 teaspoon of the vanilla extract in a mixing bowl until the filling becomes smooth, creamy, and evenly blended.
Fold 1 cup of blueberries into the cream cheese mixture with a gentle hand so the berries remain intact and are distributed throughout the filling.
Arrange half of the cubed French bread across the bottom of a 10×14-inch baking dish, forming an even layer.
Spread the blueberry cream cheese mixture over the bread layer, covering it as evenly as possible.
Place the remaining bread cubes over the cream cheese layer to create the top bread layer.
In a large bowl, whisk together the eggs, 2 cups milk, remaining 2 teaspoons vanilla extract, cinnamon, and nutmeg until the custard mixture is fully combined.
Pour the egg mixture over the layered bread, making sure the liquid reaches the bread throughout the dish.
Scatter the remaining 1 cup of blueberries evenly over the surface.
Cover the baking dish securely with aluminum foil and refrigerate for at least 8 hours or overnight.
Take the casserole out of the refrigerator 30 to 60 minutes before baking.
Preheat the oven to 375°F, or 190°C.
Bake the covered casserole for 45 minutes.
Remove the foil and continue baking for about 30 additional minutes, or until the center is set and the top is lightly browned.
Allow the casserole to cool slightly after removing it from the oven before serving.