Submerge the berries in a large bowl or sink filled with cool water. Allow any leaves and twigs (and bugs!) to float and skim off the top as much of the flotsam as you can.
Drain and transfer to a wide preserving pot; add just enough water to cover the bottom of the pot. Bring berries to a simmer over medium-high heat; reduce heat to low and simmer until softened, about 10 – 15 minutes. Press through a food mill.
Prepare canner, jars and lids.
Rinse out the preserving pot and return blueberry pulp to the pot. Add sugar, lemon, pectin and mint; bring to a simmer over medium-high heat, stirring until sugar is dissolved, then raise heat to a boil and boil, stirring only minimally, until jam reaches the set point, about 30 minutes in a wide Dutch oven.
Use the freezer plate test to tell when it's ready. Jam should puddle nicely when placed in a cold saucer for a couple of minutes.
Add 1 or 2 perfect mint leaves to each jar, then ladle hot jam into hot jars to 1/4-inch headspace. Wipe rims, affix lids and process in a boiling water bath for 10 minutes.