1. gently rinse the blackberries, mix in a baking bowl with 120 g of sugar. Cover with plastic wrap, blackberries steep for 2 hours. Next, juice with 175 ml of water in a saucepan at low temperature without making 40 min, do not stir.
2. spread a large sieve with a muslin cloth. Hang strainer inside on a baking dish. Pour in blackberries and juice. Fold gauze cloth over berries and weigh down with a few plates. Collect the juice that comes out. Stand for about 1 hour until the juice has dripped out completely.
3. Measure 1 l blackberry juice, mix remaining sugar and gelling agent. Stir juice and sugar in saucepan, stirring throughout to make bubbling 3 min. Fold in cassis, bubble briefly one more time. Pour into sterilized jars with twist-off lids, turn upside down for 5 min.
The syrup is good for flavoring sherbet or Krapfen Weisse and as a sauce for ice cream.