Preheat your oven to 350°F (175°C). In a mixing bowl, combine melted unsalted butter, granulated sugar, and flour until crumbly. Transfer to a greased 9x9-inch baking pan, pressing into an even layer. Bake for 15-20 minutes until golden.

While the crust bakes, prepare the blackberry filling. In a medium saucepan, combine blackberries, lemon juice, sugar, and cornstarch over medium heat. Stir occasionally, simmer for 8-10 minutes until thickened.

In a separate bowl, mix chopped pistachios, oats, flour, and brown sugar. Sprinkle the mixture evenly over the cooked blackberry filling.

Return the layers to the oven and bake for an additional 25-30 minutes until the topping is golden brown.

Allow to cool in the pan for 30 minutes before refrigerating for at least two hours to set before slicing.