Sift flour, baking powder and salt into a mixing bowl and cut in Crisco until mixture resembles coarse cornmeal. Add cold water. Roll into an oblong sheet 1/4 inch thick. Trim off uneven edges.
Mix blackberries with sugar and flour and spread on pastry to within 2 inches of all sides. Roll like a jellyroll and carefully tuck in ends to prevent juice from escaping. Place on a cookie sheet or baking pan with shallow sides.
Bake in a moderate oven 375ºF for 40 minutes or until pastry is lightly browned.