Preheat oven to 350°F; line a 9x13 pan with parchment.
Whisk melted butter and brown sugar until smooth.
Beat in eggs one at a time; stir in vanilla.
In separate bowl, whisk flour, cocoa, baking powder, salt.
Fold dry mix into wet mixture gently.
Coat blueberries with flour; fold into batter.
Spread batter evenly in pan; bake 28–32 min.
Cool completely in pan for 2 hours before slicing.
Slice into squares using a sharp knife.