Place all the ingredients in a saucepan except the blackberries. Bring to a gentle 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 a few 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 in the fridge to pickle for one week. They are now ready to use. Keep in the fridge for up to 4 weeks.