Begin by filling your large canning pot with water. Enough water that it will still cover the jars with 1 inch of water above them once they are place in there. This step takes the longest, so begin it right away.
Once your water is close to a boil, place your glass jars in the water for 10 minutes. After that remove and place on a clean towel.
Place your lids in a small bowl and cover with boiling water.
In your large pot add the blackberries and mash over medium-high heat. Mix together the pectin and 1/4 cup sugar. Add into the blackberries. Bring to a rolling boil. Stir in remaining sugar. Allow the mixture to thicken for 1 more minute, then remove from heat.
Ladle into your jar (use the jar funnel to make it less messy). Wipe the rims down, and place your lid on, then screw the ring on next.
Process in a water bath for 10 minutes, then remove and allow to cool on a clean towel. Not too long after you take them out you’ll begin to hear popping – that’s the jars sealing! After they are completely cool, check to make sure they all sealed. If you press down on the center of the lid it should not spring back. If it does, refrigerate the jar.
Note: Since I went with less sugar than the normal recommended amount, it may not stay preserved as long. Sugar is a natural preservative, so when you reduce that, you reduce the amount of time it will be shelf stable for.