Wash the oranges in hot water, rub dry and peel them with a zest ripper. Wash the blackberries and let them drain. Mix the blackberries, orange peel and gelling fruit sugar in a large saucepan. Bring to the boil over high heat while stirring. Let it boil for about 3 minutes while stirring. Pour jam immediately into clean, hot rinsed twist-off jars. Close the jars and leave them on the lid for about 5 minutes