Mix blackberries and lemon juice together in a large pot 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 7-10 minutes.
After the 30 minutes, mix 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 (when jam reaches 100 degrees F or you can just rub it between your fingers to make sure you don't feel any grains). If cooked longer it will be really runny.
Pour jam into clean plastic containers or glass jars and place in the fridge and/or freezer.
Jam lasts about 3 weeks in the fridge and up to 6 months in the freezer.
5-Ingredient Blackberry Freezer Jam https://www.garnishandglaze.com/5-ingredient-blackberry-freezer-jam-recipe/