Preheat oven to 350°. Spray a 10-inch wide, 2-inch deep removable-bottom tart pan with baking spray with flour. (Or you can use a 10-inch deep-dish pie plate.)
For crust: In a large bowl, toss together graham cracker crumbs, melted butter, sugar, and salt until moistened. Using the bottom of a measuring cup, press crumb mixture into bottom and up sides of prepared pan.
Bake until lightly browned, about 8 minutes. Let cool completely on a wire rack.
For filling: In a medium saucepan, bring 3 cups blackberries and granulated sugar to a boil over medium-high heat. Reduce heat, and simmer for 10 minutes, mashing berries occasionally with a fork. Place a fine-mesh sieve over a bowl. Press blackberry mixture with a spatula or a whisk through sieve. Discard seeds. (There should be about 1 cup juice.)
Place cornstarch in saucepan; whisk in blackberry juice. Bring to a boil over medium-high heat, whisking frequently. Reduce heat; simmer for 2 minutes. Remove from heat. Spoon into a bowl. Let cool completely. Cover and refrigerate ⅔ cup blackberry mixture. Reserve remaining ⅓ cup blackberry mixture.
In a small microwave-safe bowl, place 2 tablespoons cold water; sprinkle with gelatin. Let stand until softened, about 5 minutes. Microwave on high until dissolved, 10 to 15 seconds. Let stand until lukewarm, about 10 minutes.
In a large bowl, beat cream cheese and confectioners’ sugar with a mixer at medium speed until creamy. Add cream and reserved ⅓ cup blackberry mixture; beat at high speed until thickened. With mixer on low speed, add lukewarm gelatin mixture, beating until combined. Spread into prepared crust. Cover and refrigerate until chilled and set, about 6 hours. Arrange remaining 2½ cups blackberries on tart. Serve with remaining ⅔ cup blackberry mixture.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/blackberries-cream-pie-recipe/