If fresh rhubarb is used, simmer gently until tender. Combine with blueberries in a large saucepan, and sugar; mix.
Place over high heat; bring to full, rolling boil and boil hard for 1 minute, stirring constantly. Remove from heat, add pectin. (Or follow the instructions on your liquid pectin, if they are different). Stir and skim. Ladle into hot sterilized glasses. Freeze or can according to the instructions that came with your liquid pectin.
Yield: 9 half-pints