1. Preheat oven to 375°. Stir together blackberries and sugar in a medium bowl. Let stand 25 minutes or until some liquid is released and sugar turns pink.
2. Whisk together flour and next 3 ingredients in a medium bowl. Stir in melted butter until blended. Spread batter in an ungreased 8-inch square pan. Spoon blackberry mixture over batter; sprinkle with cinnamon.
3. Bake at 375° for 45 to 50 minutes or until bubbly and golden brown. Serve warm with whipped cream or ice cream.
*Frozen blackberries, thawed and drained, may be substituted.