Preheat oven to 200 degrees C. Lightly grease a baking tray.
Mix the cream cheese, sugar and cinnamon together in a bowl until thoroughly blended. Set aside.
Lightly flour the countertop and lay out the puff pastry. Cut into 6 equal squares.
Place 2 tablespoons of the cream cheese mixture onto one side of each square. Top with 6 to 8 blackberries. Brush the edges with milk then fold over to make a triangle. Press the edges together firmly to seal or crimp them with a fork.
Arrange the triangles on the prepared baking tray. Brush each one with milk and sprinkle with brown sugar.
Bake in preheated oven until golden brown; 10 to 15 minutes.