Put water, sugar, and blackberries in a medium saucepan and bring to a boil.
Once boiling, lower heat and let mixture simmer for about 15 minutes until blackberries are softening and mixture has thickened.
Remove saucepan from heat and mix chopped sage into syrup. Cover saucepan and let sit to infuse sage flavor for about 15 minutes.
Strain syrup into a bowl, pressing down on the blackberries with a wooden spoon or spatula to extract as much juice as possible.
Keep syrup in bowl or pour into a mason jar and place in fridge to cool completely before using. Syrup will continue to thicken as it cools.