Before beginning, make sure you have all the ingredients and equipment (canner, jars, lids, utensils, etc.) ready to go. Wash jars and lids in soapy hot water. Fill a water canner half full of water. Bring this to a low simmer. Fill jars with water and put on an elevated rack in canner , while water is simmering, to keep jars warm. (Or you can set jars on dish towel on a cookie sheet and keep in oven at 250 degrees for 20 minutes for same effect). At the same time, I start a teapot with water going, (so I will have boiling water for the jar lids later on).
Crush the berries one cup at a time (it is very easy if you use a potato masher). Put the finely crushed berries (5¾ cups total) into a large stock pot. Add lemon juice. Stir the box of Pectin into the berries. Bring this mixture to a full rolling boil on high heat, stirring constantly (a rolling boil is when it is boiling hard, it won't stop bubbling even when stirred). Once berries are at full rolling boil stage, stir in all of the sugar quickly (all at once). Stir well to combine.
At this point, I pour boiling water over the jar lids that are in the small bowl. Let them just sit in the hot water for 5 minutes while you finish the jam.
Bring the jam/juice/pectin/sugar mixture back to a full, rolling boil. Once it is a full rolling boil, boil it for exactly 4 minutes, stirring constantly. When time is up, remove pan from heat, then skim off any accumulated foam (and discard foam). Ladle the hot jam mixture into drained, hot, prepared jars. Fill the jars, but leave 1/8 inch headspace at the top of each jar. Insert a plastic knife into each jar a couple times to help remove air bubbles. Adjust the headspace if necessary. Use a damp cloth or paper towel to wipe down the rim and edges of the jar (you need it free of debris in order to get a good seal on the jar). Cover the jars with the hot, flat jar lid. Screw on the jar rings tightly. Lower each of the jars onto an elevated rack in the canner. The jars must be completely covered with water, and must have at least an inch of water over the top of the jar. Add more boiling water to canner, if necessary, to make sure.
Homemade Blackberry Jam https://www.thegratefulgirlcooks.com/homemade-blackberry-jam/