Makes seven 8-oz jars.
Wash, crush blackberries with potato masher, measure. Add lemon juice. Pour into large saucepan. Slowly stir in pectin and mix thoroughly. Set aside for thirty minutes, stirring every five minutes to dissolve pectin thoroughly. Add Karo syrup and mix well. Add premeasured sugar gradually. Stir well to dissolve completely. Warming to 100 degrees F will hasten dissolving. After sugar is thoroughly dissolved, pour jam into sterilized containers and seal. Store in freezer.