Preheat the oven to 350°F (175°C). Arrange the cubed brioche or challah in a single layer on a large baking sheet and toast for about 10 minutes, until edges are lightly golden but centers remain soft; remove and set aside.
In a large mixing bowl, beat the softened cream cheese and granulated sugar with a hand mixer until smooth and lump-free.
Add the eggs to the cream cheese mixture and beat until incorporated, then pour in the half and half and mix until smooth. Stir in the lemon juice, lemon zest, vanilla extract, and salt until combined.
Lightly grease a 9×13-inch baking dish. Place half of the toasted bread cubes in the dish in an even layer, then scatter half of the blueberries over the bread.
Pour half of the custard mixture evenly over the bread and berries. Add the remaining bread cubes in a single layer, top with remaining blueberries, and pour the remaining custard over everything. Press down gently with a spatula or your hands to help submerge the bread.
Cover the dish tightly with foil and refrigerate overnight, or refrigerate at least 2 hours if baking sooner.
When ready to bake, preheat the oven to 350°F (175°C). Remove the casserole from the refrigerator and let it sit while the oven comes to temperature.
Bake the covered casserole for 30 minutes. Remove the foil and continue baking uncovered for 15–25 minutes more, until a knife inserted in the center comes out clean and the internal temperature reaches about 165°F (74°C).
While the casserole bakes, prepare blueberry sauce or warm your preferred syrup if using. Serve the French toast bake warm with blueberry sauce, berry syrup, or maple syrup.