As a young person growing up on a Southern farm, I looked forward to Wild Blackberries getting ripe.They grew around the edge of our cotton and corn field so we checked them often. My mother would give me a shinny metal syrup bucket (left from making Molasses) and I would walk around the edge of the field filling it up. She made wonderful cobblers and Blackberry Jam. The seed in wild Blackberries are much smaller than the tamed ones so they were left in the jam. It was good hot or cold with creamy butter or peanut butter on a homemade biscuit.
Place well drained blackberries in a large stock pot. Add grated apple and the sugar, soft stir to combine. (the apple adds pectin to act as setting agent.) AT THIS POINT HAVE JARS STERLIZED, LIDS AND RINGS WASHED, SET JARS IN A WARM OVEN UNTIL READY TO FILL.)
Place a large plate in freezer to get real cold and serve as a jam testing agent.
With all ingredients in large stock pot all to sit at room temperature for several minutes until berries produce juice which helps prevent sticking.
Place on high heat on top of stove, bring to boil while gently stirring with a wooden spoon to prevent sticking. When rolling boil is reached turn down to medium/ high and continue to watch, stirring frequently but not continously to cause lost heat.
Get jam up to 220 degrees on cooking thermometer. Scum/ foam will form on the surface, skim it off and discard.
Continue cooking until jam begins to look jammy around edges of pot instead of watery. DO A JAM TEST: Remove the cold plate from freezer, drop a teaspoon full onto place, wait a moment then press your finger into the center. If the jam holds the print of your finger, it's done.
Be careful with jam at this point and don't let it burn on bottom. Continue to stir. Take off heat and stir, continue to stir a couple of minutes to distribute berry pieces evenly.
Use 1/2 and 1 pint size jars. with a canning funnell and a dipper, dip jam into warm jars, place flat and ring on top, tighten and set in draft free area. They should seal in a short time.
May eat warm jam with a biscuit and fresh butter.
When jars have sealed store in cool, dry area. If one doesn't seal place in refrigerator. After opening any jar later, store unused jam in refrigerator.