We love this jam on warm toast on frosty mornings.
10 cups blackberries
6 cups sugar
In a large soup pot over low heat, crush berries until soft.
Pass through a food mill to remove seeds. Return to pot and stir in sugar. Bring to a slow boil, stirring often, until sugar dissolves.
Raise heat to high and cook, stirring often, until mixture reaches 220° on a candy thermometer.
Pour into hot sterilized canning jars (leaving headspace), seal, and process for 20 minutes.