Put the rhubarb, water and blueberries in a large pot and bring to a boil over medium-high heat. Simmer for about 5 minutes, stirring often, then add the lemon juice and pectin and return to a boil. Add the sugar and bring to a full rolling boil; boil hard for 1 full minute, stirring constantly. Remove from heat and skim any foam off the surface with a slotted spoon.
Ladle into jars and seal.