Pick over the blackberries and rinse under cold water and then pat dry.
Heat the blackberries along with the sugar and vinegar in a heavy based pan over a gentle heat until the sugar has completely dissolved.
Simmer the fruit for 5 minutes then carefully remove with a slotted spoon and pack into a sterilised jar.
Raise the temperature and boil the vinegar until it becomes thick and syrup like.
Add the geranium leaf to the jar and pour over the hot vinegar.
Pop on a lid and store in a cool dark place for 1 month before using.
Once opened store in the fridge for up to 2 weeks.