Soak and rinse the blackberries to get rid of any insects etc.
Cut the apples into dice (don’t peel or core).
Place the fruits into a preserving pan or similar with enough water to barely cover.
Gradually bring to the boil and then simmer for 1 hour (uncovered).
Strain the fruit (see notes) and leave for at least 2 hrs or overnight covered with a tea towel. (Do not under any circumstances be tempted to push or squeeze the fruit, this will result in cloudy jelly).
Place 2 – 3 saucers in the freezer for testing set.
Measure the juice and stir in 450g or granulated sugar for each 500 ml of juice. (Mine measured 1.85 ltr to which I added 1.665 kg sugar).
Heat gently to dissolve the sugar, stirring constantly.
Remove any scum with a slotted metal spoon.
Once the sugar has dissolved slowly bring the pan to the boil and once at a rolling boil, time for 20 minutes, then take off the heat.
Test a few drops of jelly on a chilled saucer and pop in the fridge for a minute.
Push you finger through the jelly and if it crinkles and forms a gel like clot which you can tip over then it’s ready. If not carry on boiling for 2 minutes and repeat plate test until set achieved.
If there is any scum left then remove (there shouldn’t be now).
Then pot up into hot sterilised jars (see notes) using a ladle and jam funnel if you have one and place lids on immediately.
You may find the jelly tries to set before you get it in the jars, if this happens then just set the pan over a low heat.
Store in a cool, dark place and once opened keep in the fridge.
Unopened jars will be good for at least a year and most likely much longer.