Step 1: Sterilize your canning jars and lids by washing them in hot, soapy water, then boiling them in a large pot of water for at least 10 minutes. Keep them hot until ready to fill.
Step 2: In a large, heavy-bottomed pot or Dutch oven, combine the fresh blueberries, granulated sugar, and fresh lemon juice. Stir gently to combine. Let the mixture sit for 15-30 minutes to allow the sugar to draw out the berry juices.
Step 3: Place the pot over medium-high heat and bring the mixture to a rolling boil, stirring frequently. Once boiling, use a potato masher to gently mash about half of the blueberries for a chunky texture.
Step 4: Reduce the heat to medium-low and continue to simmer, stirring regularly, for 20-30 minutes, or until the jam thickens. Test for doneness by placing a small amount on a pre-chilled plate; if it wrinkles when pushed with a finger after 30 seconds, it's ready.
Step 5: Carefully remove the pot from the heat and skim off any foam that has risen to the surface.
Step 6: Using a funnel, carefully ladle the hot jam into your prepared hot, sterilized canning jars, leaving 1/4 inch of headspace. Wipe the rims clean, then place the lids and screw on the bands until finger-tight.
Step 7: For long-term storage, process the filled jars in a boiling water bath canner for 10 minutes. After processing, remove jars and place them on a clean towel, undisturbed, for 24 hours. Store sealed jars in a cool, dark place for up to a year. Refrigerate any unsealed jars and consume within a few weeks.