Grease a 9x13 inch ceramic or glass baking dish with butter or non-stick cooking spray.
Distribute half of the croissant cubes into the bottom of the prepared dish in an even layer.
Evenly scatter the cream cheese cubes and 1 cup of blueberries over the first croissant layer.
Top with the remaining croissant cubes and the remaining 0.5 cup of blueberries.
In a large mixing bowl, whisk together eggs, milk, heavy cream, sugar, vanilla extract, lemon zest, cinnamon, and salt until homogenous.
Pour the liquid custard mixture evenly over the croissants, ensuring all pieces are moistened.
Press down lightly with a spatula to submerge the croissants; let the assembly rest for 20 minutes to allow for optimal absorption.
Preheat oven to 350°F (175°C).
Bake for 35 to 45 minutes until the custard is set and the top croissants are golden brown and crisp.
Allow to cool for 10 minutes before slicing to ensure the internal structure stabilizes.