Prepare a small water bath canning pot and four half pint jars.

Pour the blueberries into a non-reactive pot (I used a five quart Dutch oven for this batch and it worked nicely). Using a potato masher, work the blueberries until they're mostly crushed. Add the rhubarb. Whisk the pectin into the sugar and add that to the pot as well.

Place the pan on the stove over high heat and bring the fruit to a boil. Cook, stirring regularly for 18 to 22 minutes, until the fruit starts to thicken into jam.

Once the jam has thickened satisfyingly and has reduced by at least one-third, it is done.

Funnel the jam into the prepared jars. Wipe the rims, apply the lids and rings, and process in a boiling water bath canner for 10 minutes.

When the time is up, remove the jars and set them on a folded kitchen towel to cool. When the jars have cooled enough that you can comfortably handle them, check the seals. Sealed jars can be stored at room temperature for up to a year. Any unsealed jars should be refrigerated and used promptly.