Grease a 9x13-inch pan and spread half of the bread cubes over the bottom. Top with cream cheese and 1 cup blueberries. Layer remaining bread cubes on top.
Mix together eggs, milk, and syrup in a bowl. Pour over the bread cubes in the pan. Cover and refrigerate for 8 hours, or overnight.
Remove from the refrigerator and let stand for 30 minutes.
Meanwhile, preheat the oven to 350 degrees F (175 degrees C).
Bake in the preheated oven, covered, for 30 minutes. Uncover and continue to bake until a knife inserted into the center comes out clean, about 25 minutes more.
While casserole bakes, combine water, sugar, and cornstarch for sauce. Bring to a boil until thickened. Reduce to a simmer and add blueberries and butter until cooked down. Serve on top of casserole or on the side.