Soak the blackberries in vinegar overnight with all the spices, but keep back the sugar.
The next day, strain the spiced vinegar into a saucepan, and put the blackberries into a large clean jar.
Add the sugar to the vinegar and slowly bring the mixture to the boil. Stir frequently so that the sugar dissolves. Continue to simmer until the liquid is syrupy and has reduced slightly.
Pour the hot syrup over the blackberries and seal.