Grease a 9x13 inch baking dish with butter. Arrange half of the bread cubes in an even layer at the bottom.
Scatter half of the blueberries over the bread. Drop teaspoon-sized dollops of lemon curd randomly over the layer.
Top with the remaining bread cubes and remaining blueberries.
Whisk together eggs, milk, heavy cream, sugar, vanilla, and lemon zest in a large bowl until fully combined.
Pour the custard mixture evenly over the bread. Press down gently with a spatula to ensure the top bread absorbs the liquid.
Cover tightly with foil and refrigerate overnight, or for at least 4 hours.
Preheat oven to 350°F (175°C). Remove casserole from fridge while oven heats.
Bake covered for 20 minutes. Remove foil and bake for another 20-25 minutes until the top is golden brown and the center is set.