In a large heavy saucepan, combine blueberries, rhubarb, lemon juice and water. Bring to a boil, stirring frequently. Reduce heat and simmer for 10 minutes. Stir in sugar; turn heat to high and boil vigorously until jam reaches setting point. (218 - 220F), about 10 to 15 minutes, stirring frequently. Remove from heat, skim off any accumulating foam and stir for 3 - 5 minutes. Fill sterilized jars and seal.