Combine all the ingredients for the simple syrup in a saucepan over medium heat. Stir occasionally and remove from heat after it’s thickened and sugar has dissolved.
Pour over a bowl or glass topped with a mesh strainer to separate the syrup from the unwanted lavender florets and blueberry debris. Cool fully before use.
Pour 1 tablespoon of prepared lavender syrup into a champagne flute. Follow with equal parts grapefruit juice and rosé champagne. Serve chilled.
Tip: Use the clarified simple syrup to add flavor to your cocktail, tea, or coffee. Store excess in a sealed container for future use.