Step 1: Rinse the blackberries thoroughly and pat them dry. Place 4 cups of the berries in a large saucepan, reserving the remaining 2 cups to add later for texture.
Step 2: Add the sugar, lemon juice, lemon zest, and salt to the saucepan. Stir gently and let sit for 5 minutes until the berries begin to release their juices.
Step 3: Place the saucepan over medium heat and bring the mixture to a gentle simmer, stirring occasionally to prevent sticking.
Step 4: In a small bowl, whisk together the cornstarch and cold water until smooth to create a slurry. Gradually pour the slurry into the simmering berry mixture.
Step 5: Continue to cook for 2-3 minutes, stirring constantly, until the blackberry pie filling easy glossy fresh thickens and becomes clear and shiny.
Step 6: Remove from heat and gently fold in the remaining 2 cups of fresh blackberries. Let the filling cool completely at room temperature before using in a pie or storing.