Grease a 9x13" pan with butter or spray with cooking spray. Cut bread into 1" pieces and spread evenly in prepared pan. Top evenly with blueberries. Set aside.
In a medium bowl, combine eggs, milk, maple syrup, cinnamon, and vanilla extract. Whisk until very smooth and pour over bread. Press bread down with fingers to make sure all of bread is saturated. Add a bit more milk, if needed. Cover with plastic wrap or foil and refrigerate at least 4 hours, preferably overnight.
The next morning, preheat the oven to 350º. Remove pan from fridge. Bake 40-45 minutes, or until golden brown on top.
Meanwhile, make maple-cream cheese glaze. Whisk together cream cheese and maple syrup until very smooth and set aside. To serve, drizzle glaze on top of warm French toast casserole. Top with real maple syrup, too, if desired!
Drizzle glaze on top of French toast and top with real maple syrup.