For the elderberry jelly, strip the elderberries from the stems. Rinse the berries and let them drain. Bring to a boil with water and simmer for a few minutes until bubbling. Strain through a cloth for one night.
Dilute the collected juice with water. Combine juice, blackberries and preserving sugar in a high saucepan. Bring to a boil and simmer for four minutes until bubbling.
Immediately pour into hot rinsed twist-off jars and seal the elderberry jelly with lids or preserving skins.