How to make
Pour them into a large pot and add the citric acid and water. Leave them overnight.
The next day strain them and place them back into the pot. Add the sugar, stir and boil the mixture, while stirring at all times.
Remove the foam and pour the fruit syrup into sterilized glass bottles.
As they are hot, wrap them in a blanket and leave them to cool, then store the blackberry and raspberry juice in a cool and dark place.