Spread the bread cubes on a large baking sheet or cutting board and let sit at room temperature to crisp up a bit, at least 1 hour but up to 8.
Butter a 9-inch x 12-inch baking dish and add half of the bread cubes (4 cups). Scatter blueberries on top, then add remaining bread.
In a large bowl, whisk together the eggs, milk, sugar, vanilla extract, cinnamon, and salt. Pour over the bread cubes and blueberries. Pat gently to help submerge the bread. Cover tightly and refrigerate overnight, or for at least 6 hours.
To bake, preheat oven to 350 degrees Fahrenheit. In a small bowl, mix together the 2 tablespoons granulated sugar and 1 teaspoon cinnamon. Remove casserole from refrigerator and uncover. Sprinkle the cinnamon and sugar mixture over the top.
Bake for 60 – 70 minutes, until golden brown and no longer jiggly. If it begins to brown too much toward the end of baking, cover with a piece of foil.
Cut into squares and serve with additional blueberries and maple syrup.
