Preheat oven to 350°F and line an 8-inch square pan with parchment, leaving 1-inch overhang on two sides; set aside.

Toss 1 cup fresh blueberries with 1 teaspoon of the flour (from the 3/4 cup plus 2 tablespoons) in a small bowl and set aside.

In a medium bowl whisk eggs until slightly frothy, then whisk in granulated and brown sugar until combined and glossy; whisk in vanilla.

Slowly drizzle the cooled melted butter and oil into the egg-sugar mixture while whisking to combine.

In a separate bowl whisk together remaining flour, cocoa powder, and salt until even.

Add the dry ingredients to the wet in two additions and fold gently with a rubber spatula until just combined; the batter should be thick and glossy.

If using nuts, fold in chopped toasted nuts now.

Fold the floured blueberries into the batter with two quick turns to avoid breaking them up, then spread batter evenly into the prepared pan and smooth the top; sprinkle the reserved tablespoon of berries on top if desired.

Bake at 350°F for 22–26 minutes for an 8-inch pan, starting to check at 22 minutes; the edges should look set and the center should have a slight jiggle with moist crumbs on a toothpick.

Remove from oven and cool in the pan on a wire rack for 20–30 minutes to allow carryover heat to finish setting the center.

Lift brownies from the pan using the parchment overhang and slice into 9 squares once cooled to the touch; for cleaner slices chill 30 minutes in the refrigerator before slicing.