In a large bowl combine flour, cornmeal, granulated sugar, brown sugar, baking powder, baking soda and salt. Cut in butter until flour mixture resembles coarse crumbs. Make a well in the center of the cornmeal mixture adding buttermilk, egg and lime zest. With a fork beat the egg and buttermilk until combined. Stir all the batter together just until moistened being careful not to overmix here. Toss frozen blueberries with cornstarch to coat; add to flour mixture. Stir gently 3 to 5 turns, just until berries are incorporated (do not overmix).
1-1/2 cup all-purpose flour, 2/3 cup yellow cornmeal, 2 tbsps granulated sugar, 2 tbsps tightly packed light brown sugar, 1-1/2 tsps baking powder, 1/4 tsp baking soda, 1/2 tsp table salt, 1/3 cup cold butter cut into cubes, 1/2 cup buttermilk, 1 large egg, 1-1/2 tsps lime zest, 1 cup frozen blueberries*, 1 tsp cornstarch