Grease a 9 x 13 baking dish, and spread half of the bread cubes in the bottom of the dish. Sprinkle the cream cheese pieces and blueberries evenly over the top of bread cubes. Top with the remaining bread cubes.
In a medium bowl, whisk together the eggs, milk, heavy cream, sugar, and vanilla. Pour the egg mixture evenly over the bread cubes. Cover with foil and refrigerate overnight.
Remove the casserole from the fridge 30 minutes before baking. Preheat the oven to 350. Bake 30 minutes (still covered with foil), then remove the foil and bake and additional 25-30 minutes, until cooked through.
While the casserole is baking, make the syrup by adding the blueberries, sugar, corn starch, and water to a medium saucepan. Bring the mixture to a simmer and cook, stirring frequently, until the blueberries burst and the sauce thickens. Remove from heat and let stand for a few minutes before serving.
To serve, cut squares of the french toast casserole and drizzle with the blueberry syrup.