In an 8-quart pan, add the blueberries, lemon juice, sugar, and butter; combine.
Over medium-low heat, stirring constantly, heat the mixture until the sugar is completely dissolved; increase heat to medium-high and bring the mixture to a full rolling boil, stirring constantly.
Stir in the pectin; return mixture to a full rolling boil, stirring constantly.
Boil, stirring constantly, for 1 minute; remove the pan from heat; skim off any foam; stir in the cinnamon, if desired.
To prevent the jam from separating in the jars, let jam cool for 5 minutes before filling the jars.
Gently stir jam every minute or so to distribute fruit.
Ladle the hot jam into hot jars, leaving ¼ inch headspace.
Wipe the jar rims and threads with a clean, damp cloth; cover with hot lids and apply screw rings.
Process half-pint jars in a 200° water bath for 10 minutes, pint jars for 15 minutes.