Preheat oven to 350°F (175°C) and grease a 9x9-inch baking pan.
Melt butter and chocolate together until smooth.
Stir in granulated sugar and brown sugar, then mix in eggs one at a time.
Add vanilla extract and blend until combined.
Fold in flour and salt until just incorporated.
Gently fold in blueberries.
Pour batter into prepared pan and smooth the top.
Bake for 28–32 minutes or until a toothpick comes out with a few moist crumbs.
Cool completely before slicing into squares.
Dust with powdered sugar if desired.