Using blackberry syrup will help to boost the blackberry flavor a little more. To make your own, combine equal parts by volume water and sugar, eg ¼ cup (60ml) water and ¼ cup (50g) sugar with a little under 2 parts, ½ cup (2oz/55g) blackberries in a small pan. Warm, stirring now and then, to bring to a simmer. Mash to break up the berries. Simmer a couple minutes then leave to cool, strain and then store in a bottle or jar in the fridge until needed. This will make more than you need but it will last at least a week or more and can be used for other drinks, too.