Update (8/4/18) : This syrup can be made with a variety of berries and herbs. So far, I have tried mint and basil with great results. I also increased the amount of leaves I have used from 2 to 4, because I like it better (I guess my tastes changed)! While a loyal member of Team Limeade, I also have tried this syrup with lemonade and it tastes amazing! I still occasionally use store-bought limeade, but now I typically make my own lemonade or limeade so I can control the sweetness. For a less kid-friendly version, add some vodka :-)
In a small saucepan, combine the sugar, water, and blackberries. Bring to a boil, smashing the berries with a wooden spoon. Boil until the syrup thickens slightly, about 3-5 minutes. Strain through a sieve if you don't want blackberry chunks.
In a pitcher, combine the limeade or lemonade and the blackberry + mint syrup. Start with 1/2 cup of the syrup, and add more to taste. Stir in vodka, if using. Serve over ice with mint and blackberries for garnish.