Grease a 9x13 baking dish.
In a small mixing bowl, combine the cream cheese and brown sugar; then set aside.
Cube the bread into small 1-inch cubes, and add 1/2 of the bread cubes to the bottom of the prepared baking dish.
Spoon small dollops of the cream cheese mixture onto the bread cubes until you've used it all.
Sprinkle half the blueberries over the bread and cream cheese.
Add the remaining bread cubes on top of the blueberries.
Then sprinkle on the rest of the blueberries, sprinkling them evenly all over the casserole.
In a large bowl, whisk together the eggs, milk, maple syrup, vanilla extract, salt, cinnamon, and nutmeg.
Pour the egg mixture over the bread in the baking dish, making sure to soak every single piece of bread.
At this point, you can either cover and chill the casserole for at least 4 to 6 hours or overnight; or you can go ahead and mix up the streusel topping and bake it.
When you're ready to bake the casserole, remove it from the fridge, and preheat the oven to 350° F.
Using a pastry blender, mix together the sliced butter, brown sugar, flour, and cinnamon in a small mixing bowl.
Crumble the topping over the entire French toast casserole.
Cover the baking dish with aluminum foil, and bake at 350° F for about 45-50 minutes. Then uncover and bake for an additional 15 to 20 minutes.
Allow it to cool for at least 10 to 15 minutes after baking. Then slice it and serve it with powdered sugar, maple syrup, blueberry sauce/syrup, and more blueberries.