Begin by crushing the digestive biscuits in a food processor until they resemble fine crumbs. In a mixing bowl, combine the crushed biscuits with ground pistachios, then pour in the cold heavy cream. Stir until thoroughly combined.

Spread the pistachio mixture evenly in a 9x9 inch pan, pressing down firmly with a spatula. Chill for about 15 to 20 minutes to firm up.

In a medium saucepan over medium heat, combine the fresh blackberries, sugar, and cornstarch. Stir frequently until the mixture begins to boil and thicken, around 5 to 7 minutes. Remove from heat and let it cool slightly.

After the base has chilled, beat softened cream cheese until smooth and spread it evenly over the pistachio base.

Spoon the cooled blackberry topping over the cream cheese layer, spreading it evenly. Chill for at least 4 hours or overnight.

Once set, remove the bars from the refrigerator and slice them into squares with a warm knife. Serve chilled or at room temperature.