Preheat your oven to 375°F (190°C). Grease a baking dish with a little butter to prevent sticking.
In a large mixing bowl, gently toss the blackberries with granulated sugar and lemon juice until evenly coated. Transfer the mixture to your prepared baking dish, spreading it out evenly.
In a separate bowl, combine oats, flour, and brown sugar. Mix together with a spatula or your fingers until well combined and crumbly.
Add the cold, cubed butter to the dry mixture. Use your fingers or a pastry cutter to work the butter into the oats and flour until the mixture resembles coarse crumbs with some larger clumps.
Spread the crumble evenly over the berry filling, pressing down gently to create an even layer.
Bake in the preheated oven for about 35 minutes, or until the topping is golden brown and the berry juices are bubbling around the edges with a faint sizzle.
Remove the blackberry crisp from the oven and let it cool slightly for 5-10 minutes. The topping will be crisp, and the filling will be bubbling and juicy.
Serve warm, optionally with a scoop of vanilla ice cream or a dollop of whipped cream for extra indulgence.
