Steps

1. If the berries are frozen, let them thaw.
2. In a broad, heavy-bottomed pan, simmer the blueberries, covered, for about 5 minutes. Press them though the fine disk of a food mill.
3. Combine the berry purée in the pan with the rhubarb, sugar, and lemon juice. Cook the mixture over medium heat, stirring, until the sugar has dissolved. Raise the heat to medium-high and boil the mixture, while still stirring frequently, until the rhubarb breaks down and a bit of the jam mounds in a chilled bowl, or until the temperature of the mixture reaches 221 degrees. (This should take no more than 10 minutes.) Remove the pan from the heat.
4. Skim the foam from the jam, and ladle the jam into sterilized half-pint mason jars, leaving ¼-inch headspace. Close the jars, and process them for 5 minutes in a boiling-water bath.
5. Remove the jars to a rack or pad, and let them stand undisturbed for 24 hours, after which time the jam should be well set. Store the jars in a cool, dry, dark place.