Wash and cut up the apples, but do not peel or core (the core and pips help the jelly to set). Wash the blackberries. Put the apples and blackberries together into a preserving pan, add the water, and simmer until soft. Put into a jelly bag and leave to strain overnight. Measure juice and allow 1 lb. sugar to each pint of juice. Cook over a low heat until sugar has dissolved, then boil rapidly until setting point is reached. Skim, pour into hot sterilized jars and cover tightly.