Place half the bread in a greased 11"x13"x2" baking dish. Cut the cream cheese into small cubes and place over the bread. Top with 1 cup fresh or frozen blueberries. Spread remaining bread evenly over the mixture.
In a large bowl, beat 12 eggs until whites are fully incorporated. Add 2 cups milk and 1/3 cup maple syrup. Mix well. Pour over the bread mixture. Cover with lightly greased aluminum foil and chill 8 hours or overnight.
Remove from refrigerator 30 minutes to 1 hour before baking. Cover and bake at 350 degrees for 30 minutes. Uncover and bake an additional 25-30 minutes more or until golden brown and center is set. Allow to rest for 5 minutes before cutting.
In a saucepan, mix together 1 cup sugar and 2 tablespoons cornstarch. Add 1 cup cold water and mix well. Bring to a boil and stir constantly for about 3 minutes until thickened. Stir in 1 cup fresh or frozen blueberries, reduce heat and simmer for at least 10 minutes, stirring frequently. Stir in 1 tablespoon butter until melted. Serve sauce over french toast. The sauce may be made in advance and reheated in the microwave or on the stovetop.