Most blackberry jams I have made leave something to be desired. I guess the texture is wrong. Rhubarb makes a difference. There is no fruit pectin in this recipe. You could use frozen rhubarb if you like.

BLACKBERRY JAM
2 cups fresh blackberries
4 cups rhubarb
4 cups sugar
Dash of salt

Clean blackberries and rhubarb. Cut rhubarb into 1 inch pieces. Combine with 2 cups sugar. Heat to boiling and boil 3 minutes. Add remaining sugar and salt. Boil 4 minutes or longer until thickened. Pour into hot sterilized jars and seal. Makes 6 half pint jars.