If you are canning the jams for storage, please see this earlier post for some helpful references. I have also found Putting Food By to be an extremely valuable practical resource.
Makes about 6 cups
10 minutes
preparation time
about 20 minutes cooking time
8 cups
blackberries
4 cups sugar
2 teaspoons minced fresh ginger
Put a plate in the refrigerator to chill.
Mash all the ingredients together in a large stockpot. Bring it to a boil and boil until it thickens, about 20 minutes.
To test to see if the jam will set, put a teaspoon of jam on one of the chilled plates and return it to the refrigerator for one minute. After a minute, remove it and tilt the plate. Most of the preserves should remain in the center of the plate, while some of it -- especially the more liquid portion -- will slowly spread out towards the edge of the plate. When the preserves have firmed up to this extent, skim off a bit of the foam if there is any, stir in the rest, and transfer the jam into storage jars.