Begin by preheating your oven to 350 degrees.
Rinse and dry your blackberries and then sprinkle them with the powdered sugar.
In a large mixing bowl combine the yellow cake mix, cinnamon, sugar, melted butter, sour cream, and eggs. Mix with a hand mixer for about three minutes.
Spray a bundt cake pan with non-stick spray.
Pour the cake batter in and then slowly mix in the blackberries with a spatula or wooden spoon.
Bake for about 60 minutes or until a toothpick comes out clean from close to the center.
Serve with even more fresh blackberries on the side.