Discard the centre seeds from each rockmelon half. Using a dessert spoon, carefully remove the fruit from rockmelon halves, leaving the shell intact (save the fruit for another use). Sit the rockmelon shells in bowls, so they retain their shape.
Pour 375ml (1 ½ cups) of apple juice into a small saucepan. Add the jelly crystals, whisking until dissolved. Pour jelly into a large heatproof jug. Add the remaining apple juice and whisk to combine.
Scatter 1/3 cup blueberries over the base of each rockmelon shell. Pour over enough jelly to almost cover. Refrigerate for 30 minutes until firm. Repeat layers again. Repeat a third time, filling the shells to the top and refrigerate for 3 hours or until firm. Cut into pieces and serve