Remove stems and crush fully ripe blueberries completely, one layer at a time.(Or blend them coarsely in a food processor.) Measure 4 cups of crushed fruit. Add more berries if necessary. Place berries in a very large kettle and stir in lemon juice and the Sure-Jel or Can-Jel. Place over high heat and stir until mixture comes to a hard boil. Immediately add sugar and bring to a full rolling boil; boil hard 1 minute, stirring constantly. Remove from heat; skim off foam with metal spoon; stir and skim 5 minutes to cool slightly and prevent floating fruit. Ladle into sterilized glass jars, leaving 1/2? space at top. Seal with lids. Makes about 7 cups or 3 1/2 pints of jam.