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. (218 - 220F or 103 - 104C), 10 to 15 minutes, stirring frequently. Remove from heat, skim off foam and stir for 3 - 5 minutes to suspend fruit evenly throughout jam. Fill sterilized jars and seal. Makes about 4 pint jars or 8 - half pint jars.
Source: Canadian Living