Combine the sugar and 3/4 cup water in a small saucepan. Bring just to a simmer, stirring until the sugar dissolves, then turn off the heat and add the blackberries, lightly mashing them to break them up slightly and release their color. Stir in the lemon juice and steep 10 minutes or until cool. Add the vodka and 1 1/2 cups cold water. Pour into glasses over ice. Garnish with the lemon wheels.