Combine blueberries, caster sugar and 1 cup (250ml) water in a saucepan. Stir over medium-high heat until sugar is dissolved. Bring to a gentle boil, then remove from heat immediately. Set aside to cool.
Add Gin and tonic to the blueberry mixture. Half fill 16x 80ml (1/3 cup) capacity ice-block moulds with blueberry gin and tonic mixture. Freeze two hours then fill with remaining mixture. Insert popsicle sticks. Freeze overnight.