Place four tumblers into the freezer for 30 minutes.
Dissolve sugar in 100ml water over low heat, boil for 30 seconds, then steep blackberries for 15 minutes.
Remove berries and soak gelatine in cold water for 5 minutes until soft.
Squeeze excess water from gelatine and stir into the warm syrup until dissolved.
Slowly stir in cider, pour into glasses while reserving 125ml, and freeze for one hour.
Fold the jelly to create bubbles and top with the reserved blackberries.
Gently warm the remaining jelly and pour it over the berries.
Chill in the fridge for 3 to 4 hours until fully set.