For the blackberry jam, sort the blackberries, if necessary, wash them, mix them properly with the preserving sugar and let them steep for 1 – 2 hours.
Peel the zest of the lemon thinly with a zester (only the yellow part of the zest) and add to the blackberries, squeeze the rest of the lemon and add the juice as well.
Bring everything to a boil in a saucepan and simmer for about 8 minutes. Make a jelly test. Pour some of the jam onto a small plate and put it in the refrigerator for a short time.
If the jam is too liquid, let everything simmer a little more. Skim off the foam before pouring into the jars.
Pour the jam into twist-off jars and seal tightly. Turn upside down for a few minutes.
The jars should be rinsed hot beforehand and best rinsed with rum afterwards. To do this, it is best to pour the rum into the jar just before filling the blackberry jam, close and shake well and also turn upside down.