Preheat the oven to 350°F (175°C) and grease a baking dish.
Cube or tear the bread and spread it in the baking dish, scattering the blueberries throughout.
In a bowl, whisk together the eggs, heavy cream, milk, sugar, vanilla extract, cinnamon, and salt.
Pour the custard evenly over the bread, pressing down gently so the bread soaks it up.
Dot the top with the butter pieces.
Bake for 40-45 minutes, until the top is golden and the custard is set.
Let cool slightly before serving.