Slice bread into 1" slices and arrange in a buttered 9X13" baking dish, overlapping the slices in 2 rows.
Whisk the eggs with milk, cream, Grand Marnier, sugar, vanilla, nutmeg, orange zest, and salt until well blended. Pour mixture over breaded slices, soak them evenly with the egg mixture. Cover and refrigerate overnight.
In the morning, take the baking dish out of the refrigerator and preheat the oven to 350F. Place the pan in the middle of the oven and bake until golden and slightly puffed, about 40 minutes.
Make the blackberry sauce while the French Souffle is in the oven. Place 2 cups of the berries, sugar and lemon juice in a nonreactive pan, over medium heat. Heat until the sugar dissolves and the berries give up some of their juice, Puree and press through a fine mesh strainer to remove the seeds. Return the sauce to the pan and add the remaining berries. Keep warm until serving.
To serve, put a square of the souffle on a plate. Sprinkle with powder sugar and top with whole blackberries and sauce.