In a saucepan, combine blackberry juice and lemon juice.
Sprinkle gelatin over the juice.
Bring to a slow boil, stirring constantly.
Once slow boil is maintained, add sugar.
Cook and stir about one minute to dissolve sugar thoroughly.
Pour into a bowl and refrigerate until it becomes thicker (syrupy, but not set). Whip cream and vanilla to soft peak stage.
Gently pour cream into blackberry gelatin mixture.
Fold/stir gently until an even color is achieved.
Pour into graham cracker crust