Instead of adding the sugar called for in the recipe, you can replace it with alternative sugars like erythritol, monk fruit, stevia, or allulose. Depending on which sugar replacement you use will depend on how much you add to the recipe as each one will say on the package how much is the equivalent to one cup of sugar. Keep in mind, even with using alternative sugars, the blackberry sugar won't be actually sugar-free because of the naturally occurring sugars in the fruit.