In a small saucepan, combine blackberries, sugar, and lemon juice. Cook over medium heat until the blackberries break down and the mixture thickens, about 5-7 minutes. Remove from heat and let cool slightly.

Preheat the oven to 350°F (175°C). Grease the baking pan.

In a mixing bowl, whisk together the melted butter and sugar until well combined. Add eggs and vanilla, mixing until smooth.

Sift in cocoa powder, flour, salt, and baking powder. Stir until just combined.

Pour half of the brownie batter into the prepared baking pan. Spoon half of the blackberry mixture over the batter. Use a knife to swirl the blackberry into the batter.

Pour the remaining brownie batter on top and repeat the swirling process with the remaining blackberry mixture.

Bake for 25-30 minutes or until a toothpick inserted in the center comes out with a few moist crumbs. Let cool before slicing.