Put the blackcurrants and the apple juice in a small pan, bring to a simmer and cook for 5 minutes. Add the sugar, heat gently until dissolved, then bring to the boil and simmer gently for 3-4 minutes to thicken the syrup a little. Cool, then scoop out 3 tbsp of the currants and keep to one side. Purée the rest of the currants and the syrup with a hand blender and rub through a sieve to get rid of the pips.
Beat the double cream to soft peaks, then pour in the condensed milk and vanilla extract and whisk again to soft peaks. Pour the mixture into a freezer-proof box, and drizzle over half of the blackcurrant purée. With a rubber spatula or large spoon, stir the mixture once in a figure-of-eight, just to ripple the purée through. Cover with a lid and freeze for at least 3 hours.
To serve, mix the rest of the purée with the saved blackcurrants. Scoop the ice cream into glasses and spoon over the purée.