In a large pot (off the stove), mix the blackberries and lemon juice together, and sift in the pectin a little at a time while stirring.
Once all the pectin is mixed in, set a timer for 30 minutes and give the fruit a stir every 7 to 10 minutes.
After the 30 minutes, stir in the corn syrup and then place the pot on the stove over medium heat. Stir in the sugar a little at a time and cook for about 5 minutes until the sugar is dissolved (it should be good to go when the jam reaches 100 degrees F or you can just rub it between your fingers to make sure you don't feel any grains). If it is cooked longer it will be quite runny.
Pour the jam into clean plastic containers or glass jars and place in the fridge and/or freezer.
Notes
Jam lasts about 3 weeks in the fridge and up to 6 months in the freezer.