Preheat your oven to 350°F (175°C). In a mixing bowl, cream together ½ cup of softened butter and ¼ cup of sugar until light and fluffy. Gradually mix in 1 cup of all-purpose flour until combined. Press this mixture firmly into the bottom of a greased 8x8-inch baking pan, creating an even layer. Bake the crust for 10-15 minutes until golden brown, and let it cool slightly.

In a medium saucepan, combine 2 cups of blackberries, 2 tablespoons of sugar, 1 tablespoon of cornstarch, and 1 tablespoon of lemon juice. Cook over medium heat, stirring gently, until the mixture simmers and thickens, about 5-7 minutes. Remove from heat and set aside to cool slightly.

In a separate bowl, mix together ½ cup of chopped pistachios, ½ cup of rolled oats, ¼ cup of brown sugar, and ¼ cup of all-purpose flour. Drizzle in ¼ cup of melted butter and stir until well combined.

Spoon the blackberry filling evenly over the pre-baked crust. Spread with a spatula. Sprinkle the pistachio crumble topping over the filling.

Return to the oven and bake for an additional 15-20 minutes at 350°F (175°C) until the topping is golden brown.

Remove the baking pan from the oven and allow the bars to cool completely on a wire rack for at least 1 hour before slicing into squares.