In small saucepan, combine blueberries, sugar, cornstarch, lemon rind, allspice, cinnamon and salt. Bring to a simmer over medium-high heat; simmer, covered, 3 minutes or until berries have popped and mixture has thickened. Remove from heat.
Slice ends off loaf. Slice loaf diagonally into 3/4-inch-thick slices (you should have about 18 slices).
In large bowl, whisk together eggs, half-and-half, rum, vanilla, salt, allspice and cinnamon. In 13 x 9 x 2-inch baking dish, place half the bread slices in single layer. Pour half of egg mixture over bread; press gently to submerge. Let soak 5 minutes.
Meanwhile, coat large nonstick skillet or griddle with nonstick cooking spray. Heat skillet or griddle over medium-high heat. Transfer soaked bread slices to hot griddle. Place remaining bread slices in baking dish and add remaining egg mixture.
Cook first batch of bread until golden brown, about 2 to 3 minutes per side (instant-read thermometer inserted in center of a slice should register 160 degrees F). Transfer French Toast to serving platter; cover with foil.
Cook second batch of French toast. Place on top of first batch. Dust with confectioners sugar, if desired. Serve immediately with warm Blueberry Sauce. Makes 6 servings.