Combine the rhubarb, blueberries, lemon juice, honey, and water in a saucepan over high heat. Bring to a boil, stir, and then reduce heat to low.
Maintain a gentle boil/simmer for approximately 60-75 minutes, or until the jam has set. Stir the jam occasionally and test its thickness by seeing if it coats the back of a spoon and/or moves with your spoon as your stir it.
Once the jam has set, remove from heat and transfer to an airtight container. It will last up to a few weeks in the refrigerator. See notes for freezing instructions.
Notes
Enjoy on toast, biscuits, muffins, or scones; brushed onto salmon or chicken; mixed into yogurt or oatmeal; or as the filling in my Blueberry Rhubarb Bars.
To freeze, let the jam cool before transferring to freezer-safe containers. Seal tightly. Store for up to 6 months in the freezer. Thaw overnight in the fridge or in a bowl of warm water. Use within 3 months for the best taste.