Pick the grapes from the stems. Put the grapes in a blender with the blackberries and whizz to a smooth purée.Set a fine sieve over a bowl or jug. Strain the purée through the sieve, using the back of a spoon to push as much juice and purée through the sieve as possible.
Pour into ice pop moulds and place in the freezer. Insert the ice pop sticks or spoons after 30 minutes, when the
mixture is firm enough to hold them, and return to the freezer until solid.