Wash the blackberries thoroughly, put them in a saucepan and mash them with a potato masher.
Mix the Gelfix with 2 tablespoons of sugar and add to the fruit pulp. Bring the berry mixture to the boil. As soon as it boils, add the remaining sugar and let it simmer for 3 minutes.
Fill the jam into sterile jars. Then turn the glasses upside down for 5 minutes and let them cool down.