Clean the fresh (preferably self-picked) blackberries several times in a cold water bath. Puree together with the bananas and simmer in the saucepan with the preserving sugar and vanilla sugar for approx. 3-4 minutes, fill the whole thing into prepared screw-top jars (rinse them with hot water beforehand) and close tightly. Then turn the filled jars upside down for about 5-8 minutes.
This type of jam is good for decorating cookies and as an ingredient in homemade milkshakes.