For a less sweet lemonade, reduce the amount of sugar to your preference. You can also substitute the sugar with honey or agave nectar. If you prefer a stronger berry flavor, muddle the blackberries more thoroughly before adding the boiling water. Feel free to experiment with other berries like raspberries or blueberries for a unique twist on this recipe. Adjust the amount of lemon juice to suit your taste, keeping in mind that blackberries naturally mellow the tartness of the lemons. For a sparkling lemonade, top each glass with a splash of club soda or sparkling water before serving.