Preheat oven to 325 degrees F.
Spray bottom of 11 x 6-inch baking pan (or similar size) with cooking spray. In large bowl, whisk together eggs, sugar, milk, vanilla and cinnamon.
Put half of the bread cubes into the pan, top with half of the blackberries and half of egg mixture. Repeat the layers. Let stand 15-20 minutes to allow moisture to sink into bread cubes.
Bake until top begins to brown, about 1 hour and 10 minutes. Serve warm. To add sweetness, drizzle with maple syrup, blackberry syrup or blackberry sauce just before serving.