Spray a 9-by-13-inch baking dish. Arrange the bread cubes in the dish.
Scatter the cream cheese cubes and 1 cup of the blueberries over the bread.
Whisk the eggs, milk, and maple syrup or honey. Pour it gradually over the bread so it absorbs the custard without becoming soggy. Cover and refrigerate overnight.
Stir the sugar and cornstarch in a saucepan. Add the water, stir smooth, bring to a boil, and cook for 2 to 3 minutes until thickened.
Add the remaining blueberries and simmer 8 to 10 minutes, until some berries burst while others remain whole.
Remove the sauce from the heat and stir in the butter. Refrigerate until serving.
The next morning, heat the oven to 350 degrees F. Bake the covered casserole for 30 minutes.
Uncover and bake 25 to 30 minutes more, until golden and set in the center.
Warm the blueberry sauce and spoon it over the casserole to serve.