A New England favorite, I love this Blackberry Jam recipe from an old Maine cookbook.
7 cups whole blackberries
7 cups sugar
In a large kettle put a layer of berries, then a layer of sugar, then another layer of berries and another of sugar until you have used up all the ingredients.
Let the mixture stand for an hour before you start cooking it. This allows the sugar to draw moisture out of the berries so you do not have to add any additional liquid.
Cook over medium heat stirring frequently. You should cook for about 35 minutes till the jam starts to thicken.
Pour into sterilized canning jars. Wipe the rims clean. Put on lids and metal bands and hand tighten.
Process in a water bath canning kettle for 20 minutes. Process time may have to be adjusted because of your altitude.
Remove jars from kettle and let cool.
After cooling check jar seals. Any that have not sealed properly place in refrigerator and use first.