Preheat your oven to 350°F (175°C). In a mixing bowl, combine all-purpose flour, granulated sugar, and cold butter, then blend with your fingertips until the mixture resembles coarse crumbs. Press this mixture evenly into the bottom of a lined baking pan, and bake for 10-15 minutes until golden brown and fragrant.

While the crust bakes, combine fresh or thawed blackberries, lemon juice, granulated sugar, and cornstarch in a medium saucepan over medium heat. Stir constantly for about 5-7 minutes, until the mixture thickens and becomes glossy. Remove from heat and let it cool slightly before pouring it evenly over the pre-baked crust.

In a separate bowl, mix chopped pistachios, old-fashioned oats, brown sugar, and melted butter until well combined. Use your fingers to create a crumbly texture. Once the blackberry filling has cooled a bit, evenly scatter this nutty crumble mixture over the top.

Return the pan to the oven and bake for an additional 15-20 minutes, or until the topping is golden and fragrant. Keep an eye on it to prevent burning; you're looking for a lovely golden brown top.

Once baked, remove the pan from the oven and allow the bars to cool at room temperature for about 30 minutes. Then, cover and place the pan in the refrigerator for at least 2 hours before slicing.