Crush blueberries. Stir in lemon juice. Stir sugar into fruit. Add 1/2 teas. butter or margarine to reduce foaming, if desired. Bring mixture to a full rolling boil on high heat, stirring constantly. Stir in pectin quickly. Return to a full boil and boil for exactly 1 minute, stirring constantly. Remove from heat. Skim off any foam with metal spoon. Ladle quickly into prepared jars, filling within 1/8 inch of tops. Cover. Process jars on elevated rack in boiling water for 5 minutes. Water must cover jars by 1-2 inches. Remove jars from water and cool. Let stand at room temperature for 24 hours. Store in cool, dry, dark place for up to 1 year. Refrigerate opened jellies for up to 3 weeks.