Put the blueberry syrup, lemon juice and gin in a cocktail shaker with some ice.
Cover and shake (or stir well) to cool the mixture then pour into a champagne flute, straining to hold back the ice.
Carefully top up the glass with champagne.
Garnish with a thin strip of lemon zest and blueberries on a cocktail stick, if you like, then serve.
Notes
For blueberry syrup, see blueberry lemonade recipe and follow first step to make the syrup. You can make this ahead, and as a bigger batch if you prefer. Refrigerate until needed - it should keep a week or two.You can also make this for a crowd - a bottle of champagne will make around 12 glasses, so you would need 6oz/180ml blueberry simple syrup, 6oz/180ml lemon juice and 12oz/360ml gin for the base, then stir well with ice, strain into 12 glasses then top each up with champagne.