I like to use jam sugar to make jam because it allows for a shorter cooking time than without pectin, and the jam has a fresher, more fruity taste. The vodka does not really make this a boozy jam, it just adds a little kick.
Add blackberries and jam sugar to a large heavy pot. Crack open cardamom pods with a mortar and pestle and remove the pods, discard. Finely crush the cardamom seeds and add to mixture. Stir well to combine. Cover and let sit in the fridge until the blackberries draw some juice, about 2 hours.
Remove from the fridge and slowly bring the mixture to the boil, uncovered. Place a small plate in the fridge or freezer till it's very cold.
Cook jam for about 6 minutes, stirring often. Remove any foam with a slotted spoon. Stir in the vodka. Test a small spoonful on the cold plate. If it sets, it's done. If not, cook for a little longer.
Remove pot from the heat and fill the piping hot jam into sterilised jars. Immediately close tightly with lids and leave to sit undisturbed for 24 hours, then store in a cool and dark place.