In a large heavy saucepan, combine blueberries, rhubarb, lemon rind and juice and water. Bring to a boil, stirring frequently, reduce heat and simmer, very gently, for 10 minutes.
Stir in sugar; increase heat to high and boil vigorously until jam reaches setting point 10 to 15 minutes, stirring frequently.
Remove from heat, skim off foam and stir for 3 to 5 minutes to suspend fruit evenly throughout jam. Fill sterilized jars and seal.Servings: 4 pint jars