4 cups prepared blueberries (about 1 1/2 quarts fully ripe blueberries)
1 (1.75 oz. powdered pectin
1/2 teaspoon butter
4 cups sugar, measured into separate bowl
Assemble canning equipment and jars. Fill water-bath canner half full with water. Set over high heat until it comes to a simmer.
Sanitize canning jars, lids and screw bands. Set aside.
Sort through blueberries and remove any stems. Place in colander and wash blueberries under cool water. Crush blueberries thoroughly, one layer at a time. Measure exactly 4 cups of crushed blueberries and juice into 8-10 heavy bottomed pot.
Stir pectin into crushed blueberries in pot. Add butter to reduce foaming. Bring mixture to full rolling boil (a boil that doesn’t stop bubbling when stirred on high heat), stirring constantly.
Pour in sugar from bowl and stir until sugar is incorporated. Return mixture to a full rolling boil and boil exactly 1 minute, stirring constantly. Remove from heat. Skim off any foam with metal spoon.
Immediately ladle jam into prepared jars, filling to within 1/4 inch of tops. Wipe jar rims and threads with clean towel, if needed. Cover with two-piece lids and screw bands tightly.
Place jars in rack in water bath canner. Carefully Lower rack into canner. Water must cover jars by 1 to 2 inches. Add more boiling water, if necessary. Cover canner with lid and bring water to gentle boil. Process 10 minutes.
Remove jars and place upright on towel to cool completely. After jars cool, check seals by pressing middles of lids with finger. If lids “click” or spring back, lids are not sealed and refrigeration is necessary.