Sort and wash the blueberries. Wash the apples, remove the core, cut into pieces with the skin.
Puree the blueberries and apples with the lemon juice. Put in a large saucepan and add sugar. Bring to the boil while stirring and let simmer for 8 minutes.
Immediately fill into clean screw-top jars and close, place the jars on the lid for 5 minutes, then turn them over.