Blackberry syrup that is used to flavor soda water or Italian ice can be used in place of blackberry liqueur. Use half the amount of syrup in place of the amount of liqueur the recipe calls for. Blackberry puree is another option but is best used for dessert recipes and blended drinks as its consistency is too thick for a simple drink like the blackberry martini. Make it by blending or muddling fresh or thawed frozen berries with a bit of fine sugar. Use the same amount of puree as you would liqueur.

Creme de cassis is a liqueur made by distilling black currants. It has a similar flavor profile as blackberry liqueur -- tangy, with a sweet finish. Use the same amount of creme de cassis as a substitute for blackberry liqueur. Creme de framboise or raspberry liqueur will also work well as a replacement for blackberry liqueur. The flavor of raspberries is a bit brighter and tangier than blackberries and will change the flavor in the recipe slightly but can still work in both mixed drinks and desserts.