Heat the oven to 350°F. Butter a 9-by-13-inch dish and toast the bread cubes on a sheet pan for 8 minutes.
Put half the bread and blueberries in the dish. Spoon small dollops of ricotta over them, then add the remaining bread and berries.
Whisk the eggs, milk, sugar, vanilla and salt. Pour over the bread, press gently and let soak for 15 minutes.
Cover loosely with foil and bake for 25 minutes. Uncover and bake for 15 to 20 minutes more, until the custard in the center reaches 160°F and the exposed bread is browned.
Rest for 10 minutes before scooping, preserving a few ricotta pockets in each serving.