Place the blackberries into a large sterilised glass bottle or jar and gently crush them with the back of a wooden spoon to release some of their juice.
Lightly crush the cardamom pods and put them in with the blackberries.
Pare the lime zest using a fine peeler and add the lime zest into the jar. Save the flesh of the lime for another recipe.
Pour in the gin, stirring all the ingredients to combine.
Seal tightly and store in a cool, dark place for 2 -3 weeks, shaking the jar every few days.
To make the sugar syrup place the sugar and water in a small saucepan over a low heat, stirring until the sugar has completely dissolved. Remove from the heat and leave to cool completely before adding to the blackberry gin.
Strain the blackberry gin through a conical fine-mesh sieve into a clean jug, discarding the fruit and lime zest.
Stir in the cooled sugar syrup a little at a time until the liqueur reaches your preferred sweetness. You many choose not to use all of the syrup.
Pour into 1-2 clean sterilised bottles, seal and leave to mature for 2-4 weeks before serving.