Into a saucepan over med-high heat combine sugar, blueberries, rhubarb, water, lemon juice, lemon zest, and cinnamon stick. Bring mixture to a boil then turn temperature to low. Place a tight-fitting lid over the saucepan and check after five minutes, use a potato masher to mash the blueberries and rhubarb to release the juices. Remove from heat and strain mixture through a fine-mesh sieve into a Mason-style type of jar with a lid to store.

The syrup can store in the refrigerator for up to a month.