1. Combine first 3 ingredients in a small saucepan over medium-high heat; cook for 2 minutes or until butter melts and sugar dissolves, stirring frequently. Pour sugar mixture into a 13 x 9–inch baking pan coated with cooking spray; sprinkle evenly with blueberries.
2. Remove crust from bread; reserve crust for another use. Cut bread into 1-inch-thick slices. Arrange bread slices over blueberries.
3. Combine milk, vanilla, salt, and eggs in a large bowl, stirring with a whisk until frothy; stir in liqueur, if desired. Pour milk mixture over bread. Cover and chill 8 hours or overnight.
4. Uncover pan; let stand at room temperature 30 minutes.
5. Preheat oven to 350°.
6. Bake at 350° for 40 minutes or until browned. Place a large platter or jelly-roll pan upside down over baking pan; carefully invert onto platter. Sprinkle evenly with powdered sugar.