Preheat the oven to 325°F (163°C) and line a 9x13-inch baking dish with parchment paper.

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

Whisk together cornstarch and water; add to the saucepan and stir until thickened. Remove from heat, stir in lemon zest, and fold in remaining blackberries. Allow to cool slightly before refrigerating.

Toast pecans in a skillet over medium heat for about 5 minutes, then combine melted butter, flour, and pecans.

Press mixture into the baking dish and bake for about 15 minutes. Cool before adding filling.

Whip heavy cream, adding confectioners' sugar gradually until thickened.

In a separate bowl, beat cream cheese until smooth, then fold in the whipped cream.

Spread filling over the cooled crust, then top with the blackberry mixture.

Chill in the refrigerator for at least four hours or overnight.

Cut into squares and serve with a drizzle of crème fraîche or a scoop of ice cream.