Wash the blackberries and put them in a saucepan without any liquid. Bring slowly to a boil.
Then pour the blackberries into a fine sieve and strain. That makes about 800 ml of juice.
Put the juice in a large saucepan. Dice the watermelon and add to the blackberry juice. Then mix everything finely with the hand blender. Add the preserving sugar, citric acid and the aroma and bring to the boil while stirring. Cook for 10 minutes, stirring several times. Then pour into glasses.