Coat a 9-by-13-inch baking dish with cooking spray and add the bread cubes and frozen blueberries.
Whisk the eggs, milk, sugar, lemon juice, vanilla and salt in a large measuring cup until even.
Pour the custard over the bread, press the top cubes down gently and let the dish stand for 10 minutes while the oven heats to 350°F.
Cover with foil and bake for 30 minutes.
Uncover and bake for 15 to 18 minutes more, until the center reaches 160°F, looks set rather than wet and the top cubes are browned.
Rest for 10 minutes before spooning out the soft center and crisp top together.