For the syrup, combine the water and sugar in a saucepan and heat over medium heat until the sugar has dissolved. Meanwhile, combine the lavender and blackberries in a Mason jar. Pour the hot water/sugar mixture over top. Let cool completely, cover and place in the fridge overnight.
For the drink, combine all the ingredients in a pretty glass with some ice. Adjust the amount of soda water to your taste. Garnish with a couple of blackberries and a fresh lavender sprig (if you have some) and serve.
Recipe by Swine + Wine at https://www.swineandwine.ca/refreshing-summer-drinks/