Preheat oven to 350°. Spray a 9-inch glass or metal pie plate with cooking spray.
For crust: In a medium bowl, stir together graham cracker crumbs, sugar, and salt. Add melted butter, 1 tablespoon at a time, stirring just until moistened. Press mixture into bottom and up sides of prepared plate.
Bake until lightly browned, about 10 minutes. Let cool completely.
For Filling: In a large bowl, beat condensed milk, cream cheese, and lemon zest and juice with a mixer at medium speed until smooth.
In a medium bowl, beat cream and 1 tablespoon sugar with a mixer at high speed until stiff peaks form. Gently fold whipped cream into condensed milk mixture. Spoon into prepared crust.
In the container of a blender, pulse blackberries until only small chunks remain, about 3 times.
In a small saucepan, bring blackberries and remaining 2 cups sugar to a boil over medium heat.
In a small bowl, stir together cornstarch and 2 tablespoons water. Stir cornstarch mixture into blackberry mixture, and boil until thickened, 2 to 3 minutes. Remove from heat, and let cool. Place 6 tablespoons blackberry sauce on lemon filling; reserve remaining sauce for another use. Using a knife, swirl blackberry sauce into lemon filling. Freeze, uncovered, for at least 4 hours. Thaw in refrigerator for 10 to 30 minutes before serving. Cover and freeze for up to 1 month.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/lemon-blackberry-swirled-icebox-pie/