Unfold piecrusts; press out fold lines with a rolling pin on a lightly floured surface. Cut piecrusts with a 2 1/2-inch star-shaped cutter, and fit into lightly greased miniature muffin pans.
Bake at 350° for 7 minutes or until golden. Remove from pans, and cool on wire racks.
Bring 1/3 cup water and next 4 ingredients to a boil in a heavy saucepan over medium heat. Boil, stirring constantly, 1 minute; remove from heat.
Stir together cream cheese and next 3 ingredients.
Spoon blackberry mixture into tart shells; pipe or dollop with cream cheese mixture. Sprinkle with additional powdered sugar, if desired.