Mix the rhubarb and berries with the remaining ingredients in a large saucepan and let stand for about 20 minutes until the sugar has dissolved. Bring to a boil. Just before the bubbly boils, mix with the hand blender so that the rhubarb fibers are crushed. Cook for 4 minutes, stirring constantly.
Put the gelling sample on a plate. Should set very quickly. Pour hot into prepared jars and close immediately. Makes about six 250 ml glasses.
Of course, those who have fresh berries are also in good shape. A good proportion of blueberries (approx. 40%) is important.