To sterilise the jars you can plunge them in a large pan of boiling water for 10 mins. Alternatively you can also wash them in warm soapy water, rinse them and put them on a baking tray in the oven heated up at 100°C/225°F/gas ¼ for 10 mins. Make sure everything is very clean. You do not want to do this too far in advance as the jars should be warm when the jam is poured into them.
Clean the blackberries and place them in a heavy bottom pan or preserve pan.
Peel and cut the apples into small chunks. Add on top of the blackberries with the sugar, water, cinnamon, allspice and lemon juice. Stir the mixture thoroughly.
Heat the fruits under medium to high heat until the mixture reaches boiling point. Carry on boiling jam for 15 mins stirring occasionally so that the jam does not stick to the bottom of the pan. From then on, check setting point every 5 minutes or so. Mash any chunks of apple.
To test if the jam has set, put a teaspoon of jam on one of the cold saucer. Let it cool for a minute. The jam is done when it wrinkles when pushed with your finger.
Pour the jam into sterilised jars. Cover with waxed paper and seal.
Store in a cool and dry place away from the light. Keep in the fridge once opened.