Preheat the oven to 325°F and prepare a 9x13-inch baking dish with parchment paper.

In a large saucepan, combine 2 1/2 cups blackberries, granulated sugar, and brown sugar over medium-high heat. Stir until the sugar melts and berries release their juices.

Whisk together cornstarch and water in a small bowl, then add to the saucepan, stirring until thickened. Remove from heat, stir in lemon zest, fold in remaining blackberries, and cool slightly before refrigerating.

In a skillet, toast the pecans over medium heat until fragrant.

In a bowl, mix melted butter, flour, and toasted pecans until combined. Press into the bottom of the prepared dish and bake for 15 minutes. Let cool.

Whip heavy cream until thick, gradually adding confectioners' sugar for sweetness.

In another bowl, beat cream cheese until creamy, then fold in whipped cream. Spread over the cooled crust.

Spoon chilled blackberry topping over the cream cheese filling and refrigerate for at least 4 hours or overnight before serving.