Preheat oven to 350 degrees F. Lightly grease a 2-quart round baking dish. In a small saucepan melt the butter over medium heat. Add the sage leaves and cook 4 to 5 minutes or until starting to crisp and butter is browned. Remove with a slotted spoon to paper towels. Set sage and butter aside to cool.
For the batter, in a medium bowl combine 3/4 cup flour, 1/4 cup sugar, baking powder, baking soda, and salt. In a separate bowl combine the buttermilk, cooled butter, and egg. Add wet ingredients to the dry ingredients and stir together until just moistened; set aside.
For the filling, in a large heatproof bowl combine blackberries and 3 tablespoons flour. Toss to evenly coat. In a medium saucepan combine 1 cup sugar, water, chopped sage, and lemon peel. Bring to boiling, stirring to dissolve sugar, and cook 1 minute. Remove from heat. Immediately pour over berries and toss to coat. Transfer berry mixture to prepared dish.
Pour reserved batter over the berries. Place cobbler on middle oven rack. Line a baking sheet with foil; place on bottom rack to catch any drips. Bake for 55 to 60 minutes or until the top is deep golden brown and berry mixture is thick and bubbly. Cool 30 minutes before serving. Top with fried sage leaves.