Combine blackberries, lemon juice, and simple syrup to a shaker glass. Muddle until blackberries are well broken up. Add rye whiskey and ice. Shake well until all ingredients are combined. Pour over fresh ice. Garnish with a rosemary sprig and lemon twist.
If you like your cocktails with a little more whiskey, use 2 ounces of Boundary St. Rye Whiskey. You can also top your Blackberry Whiskey Lemonade with soda water, tonic water, or ginger beer to add a little bit of carbonation.