Bring the water to a boil in a small saucepan, remove from heat, and stir in the sugar until it is completely dissolved.  Pour the sugar syrup into a heatproof container, cover and place in the refrigerator until completely chilled (about an hour). 

Meanwhile, thaw the blackberries. When the blackberries are thawed, place in a blender or food processor and process until pureed. Add the blackberry puree to the chilled sugar syrup and then strain the mixture to remove the seeds. Add the lemon juice and liqueur (if using), cover and refrigerate until the mixture is thoroughly chilled. 

Once the mixture is thoroughly chilled place in your ice cream machine and process according to the manufacturer's instructions. Once made, transfer to a chilled container and store in the freezer. When the sorbet becomes solid you may need to place it in the refrigerator for about 20 minutes so it is soft enough to serve.

If you do not have an ice cream machine you can 'still' freeze the sorbet.  Pour the mixture into an 8 inch (20 cm) or 9 inch (23 cm) stainless steel pan (sorbets will freeze faster in stainless steel), cover with plastic wrap, and place in the freezer.  After about 1 - 1 1/2 hours check the sorbet.  When the mixture is frozen to the point that a firm ring of ice has formed around the sides of the pan and there is a soft slush of sorbet in the center, remove from the freezer.  Transfer the partially thawed sorbet to the food processor and process until the mixture is a uniform slush.  This breaks up the large ice crystals that have formed on the sorbet.  (This step is what gives the sorbet its wonderful fluffy texture.)  Place the sorbet back into the pan, into the freezer, and repeat the process at least two more times at intervals of 1 - 1 1/2 hours.  After the third processing return the sorbet to the freezer for about an hour before serving so the sorbet can be firm enough to serve. 

Makes about 4 cups of sorbet.