Place all the ingredients in a saucepan except the blackberries. Bring to a slow simmer and heat until the sugar and salt have dissolved. Remove from the heat and leave to cool completely.
Gently rinse the blackberries and leave to drain for 15 minutes. Place the blackberries into a sterilised glass jar and pour over the pickling liquor. Make sure the blackberries are submerged, press down with a clean spoon if necessary. Store at room temperature for several days. They are now ready to use. Store in the fridge for 4-6 weeks.