Boil a jelly bag or tea towel in water for 2-3 minutes to sterilize.
Arrange the jelly bag on a stand with a large bowl placed underneath.
Combine blackberries, apples, water, and lemon juice in a large heavy-based saucepan.
Bring to a boil and simmer over low heat for 20-25 minutes until fruit is soft.
Pour fruit and juice into the jelly bag and let drip for 8 hours.
Sterilize jam jars in a 130°C oven for 10-15 minutes.
Measure the juice and add 450g of sugar for every 600ml of liquid.
Dissolve sugar into the juice over low heat and add crème de cassis if desired.
Boil the mixture for 10-15 minutes until it reaches setting point.
Skim off scum and pour the jelly into jars before sealing.