The chutney needs to go into sterilized pint jars. The easiest way to do this is to put them and the lids in the dishwasher on the hottest cycle to wash and dry. Take them out when you are about ready to use them and don't touch the insides of the jars with your hands. Alternatively, put the jars and lids into a very large pan of boiling water and boil for a couple of minutes before taking them out to dry on paper towels. You will also need to boil the equipment you use to take the jars out of the pan. At this stage, the jars can be placed into a low-temperature oven to dry. Make sure they are not cracked.
Put the apples, onions, sugar, vinegar, cinnamon and black pepper into a large pot over medium heat. Stir gently to combine. Bring to a boil, lower the heat and simmer for 45 to 55 minutes, stirring occasionally. The apples and onions should be lovely and soft and the liquid should be thick and syrupy.
Add the blackberries and cook for another 10 to 12 minutes.
While the chutney is hot, use a sterilized ladle to fill the sterilized jars. Put the sterilized lids on and set aside to cool. Store in a cool, dark place. The chutney will keep for up to 4 months.