Rinse the lilac blossoms thoroughly and remove all green stems. To a saucepan over medium heat, add the sugar and water. Stir until the sugar dissolves. Bring it up to a simmer and cook 2 minutes. Add the lilac blossoms and blackberries to a bowl. Pour the hot simple syrup over the lilacs and blackberries. Press the blossoms down with a spoon to submerge. Set the bowl aside on the counter and allow to steep at least two hours. Strain through a fine mesh sieve, pressing firmly on the lilacs and blackberries to release all of the flavor and color as you strain. Cover the syrup and refrigerate until ready to use.
To make a lilac gin & tonic:
Fill a highball glass with ice. Add two ounces gin. Squeeze in the juice of half a lemon. Pour 4 ounces tonic water over top. Finish with 1 ounce lilac syrup (or to sweetened to taste). Give the cocktail a quick stir before serving. Float a few fresh lilac blossoms on top of the cocktail.