Preheat oven to 350°F. Grease an 8-inch square pan and line the bottom with parchment.
Toss the blueberries with 1 tbsp flour until lightly coated; set aside.
Cream butter and granulated sugar with a mixer until pale and fluffy, about 2–3 minutes.
Beat in the egg, vanilla, and lemon zest until smooth, scraping the bowl as needed.
Whisk the 1¾ cups flour with baking powder and salt in a separate bowl.
Add dry ingredients to the butter mixture in two additions, alternating with the buttermilk. Mix just until combined.
Fold in the floured blueberries gently, keeping as many whole as possible.
Spread batter into the prepared pan and smooth the top. Sprinkle with coarse sugar.
Bake 33–38 minutes, until golden and a tester comes out clean. Cool 15 minutes before slicing.