Clean the blueberries (if using fresh) in a bath of salt water or vinegar. Rinse well.
In a small saucepan, combine blueberries, lavender, sugar, and water. Heat over medium-high heat until simmering, stirring occasionally. Once softened, gently smash the softened berries with the back of a spoon or masher. Simmer for 5-7 minutes until the berries are disintegrated and the syrup is bright purple.
Remove from heat and let rest to steep the lavender for 5-10 minutes, depending on how strong you want the flavor to come through. Be sure to taste test (the best!).
Strain out the berries and flowers through a fine-mesh sieve. Use a spoon to stir the mixture to release the syrup as needed, being careful to not let any of the fruit bits into the syrup, and discard them.
Optional: stir in a squeeze of fresh lemon juice to bring out the berry flavor if it's tasting flat; it's not about adding lemon flavor but rather enhancing the blueberry notes.
Let cool as needed and use immediately or store in an airtight container in the fridge for up to 2 weeks.