Wash and dry blackberries. Place berries in an earthenware vessel. Cover berries with vinegar. Let stand three days to draw out the juice from the berries Place cheesecloth over another bowl. Pour the mixture into the cheesecloth to strain. Let strain several hours. Discard berries. Measure the amount of liquid that is left. For every pint of liquid you will need 1 lb of sugar. In a pan boil the berry juice & vinegar mixture with sugar gently for 5 minutes. Let cool, then store or serve.