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.