Wash your half-pint jars and lids thoroughly with hot, soapy water. While they don't need to be sterilized if you're not canning for long-term storage, they should be sparkling clean. In a large, heavy-bottomed pot or Dutch oven, combine the blueberries, freshly squeezed lemon juice, and finely grated lemon zest.

Place the pot over medium-high heat. Bring the blueberry mixture to a gentle simmer, crushing the berries softly with a potato masher or a sturdy fork as they soften. Continue to simmer and mash for about 5-7 minutes, until the berries have released most of their juices and are broken down.

In a small bowl, whisk together the powdered fruit pectin with about 1/4 cup of the granulated sugar. This helps prevent the pectin from clumping. Add this pectin-sugar mixture to the blueberries in the pot, stirring vigorously for about 1 minute to ensure it's fully dissolved. Then, stir in the remaining granulated sugar and the unsalted butter.

Increase the heat to high. Bring the jam to a full, vigorous rolling boil that cannot be stirred down. Continue boiling constantly and stirring for exactly 1 minute. This rapid boil is crucial for activating the pectin and setting the jam.

Immediately remove the pot from the heat. To test if the jam is set, spoon a small amount (about 1 teaspoon) onto a chilled plate. Place the plate in the freezer for 1-2 minutes. Take it out and push the jam with your finger. If it wrinkles and holds its shape, it's ready! If it's still too runny, return the pot to high heat and boil for another minute, then re-test.

Once the jam has reached its desired set, skim off any foam that has risen to the surface (the butter helps minimize this). Carefully ladle the hot jam into your prepared clean jars, leaving about 1/4 inch of headspace from the rim. Wipe the jar rims clean with a damp cloth to ensure a good seal.

Apply the lids and screw bands until finger-tight. Allow the jars to cool completely on the counter without disturbing them. As they cool, you should hear the lids 'pop' as they seal. Once cool, check seals by pressing the center of the lid; if it doesn't flex, it's sealed. Store sealed jars in the refrigerator for up to 3 weeks, or process in a boiling water bath for longer shelf-stable storage according to specific canning guidelines.