Prepare casserole: Lightly grease a 9x13-inch baking dish. Arrange 1/2 of the bread cubes in the dish. Spread the cream cheese evenly on top of the bread cubes. Sprinkle blueberries over the cream cheese, then top with remaining bread cubes.
Whisk eggs, milk, vanilla extract, and syrup together in a large bowl until well-combined; pour over the bread cubes. Cover and refrigerate overnight.
Remove casserole from the refrigerator about 30 minutes before baking. Preheat the oven to 350 degrees F (175 degrees C).
Sprinkle cinnamon and white sugar over the top of the casserole.
Bake casserole in the preheated oven, covered, for 30 minutes. Uncover, and continue baking until center is firm and surface is lightly browned, about 25 to 30 minutes.
Meanwhile, prepare blueberry sauce: Mix sugar, water, and cornstarch together in a medium saucepan; bring to a boil and cook, stirring constantly, 3 to 4 minutes. Stir in blueberries, reduce heat to low, and simmer until all the blueberries burst, about 10 minutes. Stir in butter.
Serve portions of casserole on plates and pour warm syrup over top.
Servings 10
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.