Preheat oven to 425 degrees.
Combine blackberries with the 1 cup of sugar and set aside.
Mix together the flour, baking powder, salt, and the cinnamon or nutmeg. Cut in the butter until crumbly and add milk a little at a time until your dough forms a ball (this can also be done in the food processor). Turn dough out onto a floured surface and roll into a rectangle approximately 9 inches by 11 inches. Place 1/2 of the berries over the dough and roll up jelly roll style, pinch the seams. Place the roll in a 9 x 13 inch baking pan. Brush the roll with the 1 tablespoon of milk and sprinkle with sugar. Put the remaining berries around the roll. Bake for 30 minutes. Cut into slices and serve with whipped cream.