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 to store in the fridge or freezer.
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.