In a medium saucepan, combine 1 cup of granulated sugar with 1 cup of water. Bring to a boil over medium heat. Remove from heat and add 2 cups of lilac flowers (not stems or leaves) and 4 blackberries. Steep for 2 hours. Strain in fine mesh sieve.
For the Cocktails
Fill a glass with ice. Add gin, grapefruit juice and lilac simple syrup. Top with tonic. Stir and serve immediately. Enjoy!