Preheat the oven to 350°F (175°C).
Grease the 9x13 inch baking dish with some of the melted butter.
In the prepared baking dish, spread half of the cubed bread evenly on the bottom and sprinkle with half of the blueberries. Repeat with the remaining bread and blueberries.
In a mixing bowl, whisk together the eggs, milk, granulated sugar, vanilla extract, cinnamon, and salt until well combined.
Pour the egg mixture evenly over the bread and blueberries, ensuring that all the bread is coated. Gently press down with a spatula to absorb the liquid.
Drizzle the remaining melted butter over the top of the casserole.
Cover the baking dish with aluminum foil and let it sit in the refrigerator for at least 30 minutes or up to overnight.
Bake the casserole in the preheated oven for 30 minutes covered, then remove the foil and bake for an additional 15 minutes, or until the top is golden brown and a toothpick inserted in the center comes out clean.
Allow the casserole to cool for a few minutes before cutting into squares. Serve with powdered sugar if desired.