1. Prepare Streusel: Stir together first 3 ingredients in a large bowl; add flour and pecans, and stir until blended. Let stand 20 minutes or until mixture is firm enough to crumble into small pieces.
2. Meanwhile, prepare Filling: Preheat oven to 375°. Stir together peaches and next 3 ingredients in a large saucepan; bring to a boil over medium-high heat. Reduce heat to medium, and boil, stirring occasionally, 6 to 7 minutes or until juices have thickened. Remove from heat, and stir in blackberries. Spoon mixture into a lightly greased 9-inch square baking dish. Crumble streusel over hot peach mixture.
3. Bake at 375° for 30 to 35 minutes or until bubbly and golden brown.