Wash, stone and chop peaches. Sort the blackberries, wash them carefully and let them drain.
Mix the fruits with the preserving sugar and citric acid in a large saucepan and bring to the boil. Let simmer for 4 minutes while stirring, remove from heat and stir in walnuts.
Immediately pour jam into twist-off jars that have been rinsed with hot water, close tightly and let stand turned on the lid for 5 minutes. Turn several times as it cools so that the nuts are evenly distributed in the glass.