Preheat the oven to 350°F (175°C).
Grease a 9x13 inch baking dish with butter.
In a large mixing bowl, whisk together the eggs, milk, heavy cream, granulated sugar, vanilla extract, ground cinnamon, and salt until well combined.
Place the cubed bread into the greased baking dish and sprinkle the blueberries evenly over the bread.
Pour the egg mixture evenly over the bread and blueberries, making sure all the pieces are soaked. Press down gently to help the bread absorb the liquid.
Cover the baking dish with foil and let it sit in the refrigerator for at least 30 minutes (or overnight for best results).
Remove the foil and bake the casserole in the preheated oven for 30-40 minutes, or until it is puffed and lightly golden on top.
Let it cool for a few minutes before slicing into squares. Serve warm, dusted with powdered sugar if desired.