Put 1 pound blueberries, lemon juice, cornstarch and sugar in a pot together and mix thoroughly until cornstarch is soft and dissolved with no remaining clumps. Cook over medium heat while stirring constantly, scraping bottom with a wooden or heatproof rubber spatula.
When mixture starts to thicken, mash blueberries against sides of pot with your spatula to make sure most of them burst by time cornstarch activates. When mixture is at a rolling boil, add gin and cook for just a minute or 2 more. Remove from heat when filling is a deep purple color and no longer tastes pasty from cornstarch. Taste filling to determine if it needs more sugar, acid (which would require more lemon juice) or fresh blueberry flavor (which will come from adding more gin). All of these factors change with each batch of blueberries.
Immediately stir in remaining fresh blueberries and pour filling into cooled pie crust.
Chill pie for at least an hour before slicing. Serve with whipped cream.