Cut grapefruit crosswise into halves. Carefully cut around sections and scoop sections out: set aside.
Coarsely crush the berries. In a saucepan, combine the blackberries, sugar and cornstarch. Cook mixture over medium-high heat stirring constantly. Bring mixture to a boil; cook 1 more minute.
Remove from heat. Stir in gelatin and blend well; chill 30 minutes.
Drain extra juice from grapefruit sections, reserving for another use. Fold grapefruit into berry mixture and spoon into baked pie shell. Return to refrigerator and chill for 3 hours. At serving time, top each pie slice with a dollop of whipped cream and garnish with a thin slice of grapefruit. Serves 6 to 8.