Preheat oven to 350 degrees, and spray a 10-inch cast-iron pan with nonstick spray.
In a small bowl, toss blueberries with 1 scant tablespoon of flour. This keeps the berries from sinking to the bottom of the cake.
In a large bowl, using a hand mixer, cream together the butter and sugar until light and fluffy, about 3 to 4 minutes. Beat in the the eggs, 1 at a time. Mix in buttermilk, vanilla and lemon zest. Slowly sift in the remaining flour, baking powder and salt, then beat until just incorporated.
Fold in the berries with a rubber spatula, then spoon the batter into the prepared skillet. Bake for 45 minutes. Let cool for before serving. Top with whipped cream and more berries, to serve.