Place half of the bread cubes in a lightly greased 9x13 inch baking pan. Sprinkle cream cheese on top of bread cubes. Top with 1 cup blueberries and remaining bread. In a large bowl, beat together eggs, milk and maple syrup. Pour egg mixture over bread. Cover pan and refrigerate overnight.
The next morning, remove pan from refrigerator 30 minutes before baking. Preheat oven to 350 degrees F (175 degrees C).
Cover pan with aluminum foil and bake in preheated oven for 30 minutes. Uncover pan and bake for an additional 30 minutes, until golden brown and center is set.
To make Sauce: In a saucepan, combine sugar and cornstarch, add water. Boil over medium heat for 3 minutes, stirring constantly. Stir in blueberries and reduce heat. Simmer 8 to 10 minutes, or until the berries have burst. Stir in butter until melted. Serve the sauce over squares of french toast.