Preheat oven to 400°F (200°C) and blind-bake the pie shell for 8-10 minutes until lightly golden.
In a large bowl, whisk flour, sugar, and salt. Then add eggs, melted butter, grapefruit juice, Campari, and heavy whipping cream.
Strain the mixture into a bowl and whisk in grapefruit zest.
Pour the custard into the pie shell on a rimmed baking sheet, then transfer to the lower rack of the oven.
Reduce temperature to 325°F (160°C) and bake for 35-40 minutes until slightly puffed and center is set.
In a pot, combine sugar, Campari, and water. Bring to a boil, then simmer for 3 minutes. Cool and add blackberries and grapefruit segments.
Serve pie slices topped with Campari syrup and whipped cream.