Prepare a jelly bag or tea towel by boiling in water for 2-3 minutes. Wring well and leave to cool. Arrange the jelly bag on a stand or up-turned stool with a large bowl beneath, ready for the fruit juice to drip through.
Place the blackberries, apple, water and lemon juice in a preserving or large, heavy based saucepan.
Bring to the boil, then simmer over a low heat for 20-25 minutes or until the fruit is completely soft.
Tip the soft fruit and juice into the jelly bag and leave to drip for 8 hours or until all the juice has been released.
Prepare the jam jars by washing in hot soapy water and leaving to dry and warm in a cool oven - 130C/250F/Gas ½ for 10-15 minutes.
Measure the juice. For every 600ml/1 pint weigh 450g/1lb sugar. Put the juice and sugar back into the clean preserving pan, heat over a low heat until all the sugar has dissolved, add the crème de cassis, if using. Bring to the boil and simmer for 10-15 minutes or until setting point is reached.
Skim away any scum from the top of the jelly and fill the jam jars to the brim. Cover, seal and label. Store in a cool, dark place until required.