Prepare the Pan: Preheat the oven to 350°F. Line an 8 × 8-inch baking pan with parchment paper, leaving enough paper to help lift the brownies out after baking.
Melt the Chocolate: Add about 1 inch of water to a small saucepan and heat over medium heat until it begins to boil. Place a heatproof glass or metal bowl, or a larger saucepan, over the smaller saucepan. Add the butter and dark chocolate, then stir as they melt. Finish Melting: Continue stirring until the butter and chocolate have completely melted together into a smooth mixture. Remove the bowl from over the saucepan and allow the mixture to cool slightly.
Dissolve the Sugar: While the chocolate mixture is cooling, add the brown sugar and stir until the sugar is dissolved into the mixture.
Add the Wet Ingredients: When the mixture is warm but no longer hot, add the vanilla extract and room-temperature eggs. Beat well until everything is thoroughly combined.
Combine the Dry Ingredients: Add the salt, all-purpose flour, and cocoa powder to the chocolate mixture. Stir until the batter is evenly combined and no streaks of flour remain.
Fold in the Blueberries: Gently fold in half of the washed and dried blueberries, taking care to distribute them throughout the batter.
Fill the Pan: Transfer the brownie batter to the parchment-lined baking pan and spread it into the pan.
Bake the Brownies: Bake at 350°F for 30–35 minutes, until the brownies are baked through and set.
Cool Before Serving: Allow the baked brownies to cool before removing them from the pan and slicing into 8 brownies.
