In a saucepan, combine sugar, corn starch, salt, corn syrup, and 1 cup water. Stir and place over medium heat. In a separate bowl, pour the package of gelatin over 3 tablespoons water to let bloom. Raise heat on the stove and bring mixture to a low boil. Cook until thickened, 2-3 minutes.
Once thickened, remove from heat. Add bloomed gelatin and peach jello, stirring to combine. Fold in prepared peaches and blackberries.
Pour filling into a pre-baked pie shell and place in the refrigerator for 4-8 hours until completely chilled.
Video
Notes
If you want to use a 9-inch pie shell, reserve 1 cup of the thickened fruit and filling in a separate bowl and enjoy separately.