Prepare the blueberries: Crush the blueberries with a masher until they are broken down. Avoid smashing the blueberries to the point where they are puréed. Have the sugar measured out into a separate bowl so it's ready when it's time to add to the mixture.

Cook the blueberries: In a heavy-bottom kettle, add the blueberries, Sure-Jell, butter, and lemon zest. Heat the mixture while stirring to combine. Allow the mixture to come to a rolling boil, which is a boil that does not cease when stirring, 8-12 minutes.

When the blueberry mixture is at a rolling boil, add in the sugar. Allow the mixture to come back up to a rolling boil and let it boil for exactly 1 minute.

Ladle the blueberry jam into sterilized jars, leaving ¼ inch of headspace. Clean off the rim of each jar to ensure none of the jam will interfere with the jar's sealing. Place a prepared lid and ring on each jar. Tighten to finger-tip tightness.

Water bath the jars: Lower the jars into a prepared water bath canner fitted with a rack. Bring to a boil and boil for 10 minutes. Then, remove the jars from the canner and set them on a cooling rack. The jars should seal quickly. Allow to cool for 12 hours before checking the seals.