Place blueberries and sugar into a medium bowl. Crush the berries, and then stir to combine.
Cover bowl with plastic wrap and place in refrigerator. Allow to macerate for up to 2 days.
Meanwhile, place lavender into a nonreactive container, cover with cider vinegar, and store in a cool, dark place for up to 2 days. (The fridge is fine, but unnecessary.)
Position a fine-mesh strainer over a small bowl and pour blueberry mixture through to remove solids.
Strain vinegar mixture over the same mesh strainer, into same bowl as blueberry syrup. Allow to combine.
You may have some sugar clinging to the berry solids in the strainer. If so, set the strainer with the solids over another small bowl. Pour the syrup-and-vinegar mixture over the solids to wash the sugar into the bowl. Repeat as needed.
Pour syrup-and-vinegar mixture into a clean mason jar. Cap it, shake it well to incorporate any undissolved sugar, and place in the refrigerator for a week before using.
Discard the solids or save them for another use.