Lightly grease a 9x13 inch glass baking dish. Arrange half the bread cubes in the dish, and top with the cream cheese cubes.
Drain juice from 2 cans of Blueberries and reserve the juice.
Arrange the drained Blueberries from 1 can over the cream cheese, and cover with the remaining bread cubes.
In a large bowl, mix the eggs, milk, vanilla extract, and maple syrup. Pour mixture over the bread cubes. Cover and refrigerate overnight.
Take the pan out of the refrigerator approx. 30 minutes before ready to bake. Preheat the oven to 350 F.
Cover with foil and bake 30 minutes. Uncover, and continue baking 25 to 30 minutes, until the center is firm and the top is lightly browned.
In a saucepan, mix the blueberry juice from both cans and the cornstarch. Bring to a boil, and stir continually for about 3-4 minutes until the mixture thickens slightly. Adjust amount of cornstarch based on how thick you like the sauce.
Mix the drained blueberries from the second can into the sauce and simmer a few more minutes until heated through. Stir in the butter, and pour over the baked french toast.