This is a good jam to make from the last of the rhubarb crop which will be finishing when the blackberries come into season. Wash the berries and remove any stems and unripe berries. Simmer in the water until tender and sieve to remove all seeds. Wash and cut up the rhubarb and put into the blackberry pulp. Simmer until soft and then weigh the fruit. Allow 1lb/500g sugar to each 1lb/500g fruit.