Spray a 9" x 13" pan w/ cooking spray. Place 1/2 the bread in pan, in bottom. Top w/ 2 cups blueberries. Scatter cream cheese cubes all over. Cover w/ remaining bread.
In lg bowl, beat eggs, syrup and milk until combined. Pour over casserole. Cover, chill overnight.
Heat oven to 350. Bake strata 30 mins, uncover, bake 30 mins more. In saucepan, combine sugar, cornstarch and water, bring to boil, cook 5 mins. Place remaining blueberries in pan, continue to cook 10 mins, stirring often. Remove from heat. Stir in butter. Serve sauce w/ strata.