Grease a 9x13 inch ceramic baking dish with unsalted butter.
Distribute half of the brioche cubes evenly across the bottom of the dish.
Scatter the cubed cream cheese and 1 cup of blueberries over the first bread layer.
Top with the remaining brioche cubes and the remaining 1 cup of blueberries.
In a large bowl, whisk eggs, milk, heavy cream, sugar, lemon zest, lemon juice, vanilla, cinnamon, and salt until the emulsion is homogeneous.
Pour the custard mixture slowly over the bread, ensuring total saturation of all cubes.
Cover the dish tightly with foil and refrigerate for a minimum of 4 hours to allow the starch to hydrate.
Preheat a convection oven to 350°F (175°C).
Bake covered for 30 minutes, then remove foil and bake for an additional 15 minutes until the internal temperature reaches 160°F (71°C) and the surface is golden brown.
Rest the casserole for 10 minutes at room temperature to allow the custard to set before slicing.