Preheat your oven to 350°F (175°C).
Grease your 9x13 inch baking dish with butter or non-stick spray.
In a large mixing bowl, combine the cubed French bread and blueberries. Carefully fold in the blueberries to avoid smashing them.
In another bowl, whisk together the eggs, milk, granulated sugar, vanilla extract, cinnamon, and salt until well combined.
Pour the egg mixture over the bread and blueberries, ensuring all pieces are soaked.
Drizzle melted butter over the top of the mixture.
Cover the dish with plastic wrap and let it sit in the refrigerator for at least 30 minutes, or overnight for best results.
Remove the casserole from the refrigerator and allow it to sit at room temperature for about 10 minutes before baking.
Bake in the preheated oven for 35-45 minutes, or until the top is golden brown and the center is set.
Allow to cool for a few minutes, then serve warm, dusted with powdered sugar if desired.