Cut the apple into small pieces and place in a medium saucepan. Add blackberries and water and bring to the boil. Boil for 15 minutes or until the fruit breaks down.
Using muslin and a sieve, separate the hot juice from all the bits and pieces. Discard anything caught in the muslin and sieve. Allow the juice to drain into a clean pan.
Add the guava paste and sugar to the juice and bring to the boil. Begin testing for the setting point by placing a small drop of jelly onto a cold plate. If it wrinkles when you push it with your finger, it is ready. Transfer to sterilised jam jars, seal and leave undisturbed for 8 hours to set.