In a medium saucepan, bring 4 cups water and sugar to a simmer, stirring occasionally. Remove from heat, and add lavender and blueberries; steep in liquid for 20 minutes.
Strain through a fine-mesh sieve, discarding lavender and blueberries. Refrigerate until completely cool. Syrup will keep in an airtight container for up to 1 month.
Recipe by Southern Lady Magazine at https://www.southernladymagazine.com/lavender-french-75-recipe/