Score the membrane of the ribs with a knife. In small bowl, mix chili powder, garlic, salt and pepper. Rub into both sides of ribs. Wrap ribs tightly with foil and place on rimmed baking sheet. Bake for 3 hours.
Meanwhile, puree remaining ingredients in blender until almost smooth. Strain puree into medium saucepan. Cook on medium-high heat, stirring frequently until reduced to about 1 cup. (About 5 minutes) Season with salt and pepper to taste.
Open foil wrap on ribs. Brush sauce generously over the ribs. Continue baking uncovered for about 15 minutes until sauce browns and forms a thick glaze.. Cut the ribs apart and serve with the remaining blackberry sauce.