Butter a 9x13-inch baking dish. Arrange the bread cubes in the dish in an even layer.
In a large mixing bowl, whisk together the eggs, heavy cream, milk, granulated sugar, lemon zest, lemon juice, vanilla extract, and salt until smooth and fully combined.
Pour the custard mixture evenly over the bread cubes, pressing down gently with a spatula to ensure all pieces are submerged. Let stand for 30 minutes at room temperature, or cover and refrigerate overnight.
Preheat oven to 350°F (175°C). Toss the blueberries with 1 teaspoon flour in a small bowl, then scatter them evenly over the soaked bread.
Drizzle the melted butter over the top and sprinkle with coarse sugar for a golden, crunchy crust.
Bake uncovered for 45-55 minutes, until the top is golden brown and the center jiggles only slightly when gently shaken. A knife inserted in the center should come out mostly clean.
Remove from the oven and let cool for 10 minutes before serving warm, or cool completely and serve at room temperature.
