Place the blackberries in a large saucepan with half the water and simmer gently until soft. Put the apples in a preserving pan with the remaining water and simmer gently until soft. Pulp with a wooden spoon or a potato masher. Add the blackberries and sugar to the apple pulp, stirring until the sugar has dissolved, then add a knob of butter, bring to the boil and boil rapidly, stirring frequently, for about 10 minutes. Test for a set and when setting point is reached take the pan off the heat and remove any scum with a slotted spoon. Pot and cover the jam in the usual way.
I quote this recipe from The Good Housekeeping Complete Book of Home Preserving with permission from the publishers.
© 1986 All rights reserved. Published by Macmillan.