Puree two cups of the blackberries in a blender until liquid. Strain out the seeds, and pour into a saucepan. Whisk in 1 1/4 cups sugar. Cook over a medium heat until boiling, stirring constantly and scraping bottom. Keep at a boil for 5 minutes, and then remove from heat.
Mix cornstarch and brandy in a cup. When well mixed, whisk into pan; stir until mixture returns to a boil. Remove from heat. Stir in the nutmeg and cinnamon. Check the consistency of the mixture; it should be like a thin pudding. If necessary, add cornstarch and water mixed at a 2:1 ratio to thicken.
Fill the pie shell with the remaining two cups of blackberries. Pour the blackberry puree over the fresh berries, covering evenly. Use either strips of dough criss crossed to cover, or cover with a solid sheet of dough with slits cut in it. Dust with sugar on top.
Bake in a preheated 400 degrees F (205 degrees C) oven for 35 minutes, or until crust is browned.