Delicious and easy.
4 cups blackberries
4 cups sugar
2 teaspoons fresh lemon juice

Combine all ingredients in a large pot. Once the berries soften, crush them with a potato masher (or whatever works for you).
Bring to a boil. Reduce to a simmer.
While simmering continuously stir for 10 minutes.
Test the set by dipping a metal spoon in the jam. Set the spoon in the freezer for 5 minutes. If you are happy with the consistency, ladle into sterilized jars. Water bath for 10 minutes.