Add the apples, blackberries and measurement water to a preserving pan, bring to the boil, then cover and cook gently for 30–40 minutes, stirring and mashing the fruit from time to time with a fork, until soft.
Leave to cool slightly, then pour into a scalded jelly bag suspended over a large bowl and allow to drip for several hours.
Measure the clear liquid and pour back into the rinsed preserving pan. Weigh 500 g (1 lb) sugar for every 600 ml (1 pint) of liquid, then pour into the preserving pan. Heat gently, stirring from time to time, until the sugar has dissolved.
Bring to the boil, then boil rapidly until setting point is reached (10–20 minutes). Skim with a draining spoon or stir in butter if needed. Leave to cool for 5–10 minutes, then stir in the mint.
Ladle into warm, dry jars. If the mint begins to float, leave to cool for 5–10 minutes more, then stir very gently with a teaspoon to redistribute the mint. Cover with screw-top lids, or with waxed discs and cellophane tops secured with elastic bands. Label and leave to cool.
To serve, this jelly is a great accompaniment for drop scones and butter.