First, prepare the fruit. Crush completely about 1 1/2 quarts fully ripe blueberries. Measure 4 cups into a very large saucepan. Add lemon juice.
Then make the jam. Add powdered pectin to fruit in saucepan and mix well. Bring to a hard boil over high heat, stirring constantly. At once stir in sugar. Bring to a full rolling boil and boil hard 1 minute, stirring constantly.
Remove from heat and skim off foam with metal spoon. Then alternately stir and skim for 5 minutes to cool slightly and to prevent floating fruit. Ladle quickly into glasses. Cover with paraffin.