Wash blueberries and pat dry. Measure out 1 cup blueberries and set aside. Set clean jars on the counter along with lids.
In a large pot, whisk together sugar and cornstarch to break up lumps. Add 6 cups blueberries, water, and lemon juice.
Set over medium heat and cook, stirring occasionally, until just starting to bubble and thickening. This will take about 10 minutes.
Remove from heat and stir in reserved 1 cup blueberries and butter until butter is completely melted.
Pour pie filling into jars, leaving ¼-inch space at the top. Screw on lids lightly and let cool to room temperature, then place in the refrigerator until ready to use. *For water bath canning, see notes below.
Makes about 3 pints.
To make this recipe for pantry storage, swap the cornstarch for ? cup ClearJel. Process in a water bath canner for 30 minutes.
To make a pie, use 2 jars (4 cups) filling per 9-inch pie. Bake at 400°F for 50-60 minutes.