The night before you want to make the jam or the morning of. Add the rhubarb, blueberries, sugar and lemon juice to a large pot, stir to combine. Let macerate for 30 min. to overnight. Sterilize jars and lids. Place a small plate in the freezer.

When ready to make the jam place the pot with the fruit in it on the stove. Bring to a boil on medium-high heat and boil hard for 15-25 minutes or until you reached the setting point, stirring occasionally. At the setting point, the foam will have subsided and the bubbles will get larger looking like fish eyeballs.

Place a small amount of the jam (1 tsp.) on the freezer plate and place the plate back in the freezer for a few minutes to cool. If the jam seems very close then remove the pot from the heat while the plate is in the freezer. If the jam wrinkles when pushed with your finger the jam is done. If not return to a boil for 5 min. and repeat. Careful as the jam thickens it can start to burn so stir more frequently.

Ladle jam into hot jars to within 1/4" of the top. Remove air bubbles and wipe the rim with a clean cloth. Place lids on and screw to tighten until finger-tip tight only. Place in a water bath canner, submerged in 1" water above jar and process for 10 min. or according to altitude.

When the jam has cooked for its allotted time removed the jars to a towel-lined counter to cool. You should hear the popping of the lids as they seal. That is a very satisfying sound. If any jars don't seal refrigerate and eat right away.