Heat the oven to 425°F. Brush an 18-by-13-inch rimmed sheet pan with the oil, including the corners and sides.
Whisk flour, sugar, baking powder, baking soda and salt in a bowl. In another bowl whisk buttermilk, eggs, melted butter and vanilla.
Fold wet ingredients into dry just until no dry flour remains; leave small lumps. Spread batter evenly across the prepared pan and scatter blueberries over the surface, pressing them just into the batter.
Bake for 14 to 18 minutes until the surface springs back lightly and a toothpick in the center comes out without wet batter.
Rest for 5 minutes, cut into 12 equal squares and serve 2 squares per portion with one-sixth of the maple syrup.