Cut grapefruit crosswise into halves. Carefully cut around sections and scoop sections out; set aside.
Coarsely crush the berries.
In a medium saucepan, combine the blackberries, sugar, and cornstarch.
Cook mixture over medium-high heat stirring constantly. Bring mixture to a boil, then cook 1 more minute
Remove from heat. Stir in the 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 at least 3 hours.
At serving time, top each pie slice with a dollop of whipped cream and garnish with a thin slice of grapefruit.