Alcohol-free
Replace the gin and elderflower liqueur with a botanical spirit and elderflower cordial.

Gin, cucumber and elderflower, topped with soda and served long over ice.
Cocktails
Elderflower brings the sweetness, cucumber keeps it fresh, and a short pour of lemon stops the drink becoming too soft. The lemon also shifts the blue a little towards turquoise.
Elderflower cordial is usually sweeter than elderflower liqueur. If that is what you have, start with 10 ml and taste before adding more.
Clear gin lets the colour show. Dark spirits, bitters and coloured syrups won't.
With 15 ml lemon, the drink will sit somewhere between blue and turquoise. Cut the lemon to 10 ml if colour matters more than sharpness.
Replace the gin and elderflower liqueur with a botanical spirit and elderflower cordial.
Add four mint leaves with the cucumber and shake gently to keep the flavour fresh rather than bitter.
Replace the soda with 75 ml dry sparkling wine and serve in a wine glass.
For ten drinks, batch everything except the soda:
Whisk the blue spirulina into the water first. Gently press the cucumber in the remaining ingredients, strain, combine with the blue mixture, and refrigerate.
Use 105 ml batch per drink and top with 60 ml soda. Make and use the batch on the same day; fresh cucumber loses its edge quickly.

Blue spirulina powder
Choose a pack