Wash your blackberries and pick out any impurities. Natural blackberries usually don't have insects and worms in them (compared to raspberries) so the washing should be quick. Weigh the berries and place them into a big cooking pot.
Add the whole sugar amount to the berries. Mix the content well.
Keep the pot with the fruits on the stove, cook on slow heat for about 45 mins, while stirring at times, until you can see a rolling boil. At that point, the jam should be ready and you can take it from the heat. Test if the jam is set by dropping some on a cold plate, the jam is ready if it doesn't run. If it runs, keep for some more time to cook on low heat and repeat the test until you have the correct consistency. Setting temperature with the help of a thermometer is 105 Celsius/220 Fahrenheit.
At this point you can either mash the cooked blackberries quickly or keep them in whole pieces. If you mash them, and you don't like the seeds, you have the option too to strain the jam. I did not strain since we enjoy the seeds in the jam.
Before you fill the jars with the jam, drop little rum on into the lid. That will help in keeping the jam germ and mold growth free.
Fill your clean jam jars with the blackberry jam, close well with the lid and turn the jar upside down. Keep them upside down for a few hours and then store them in a dry and cool place and keep away from direct sunlight. If you live in a humid tropical climate, please store the jam in the fridge.
Notes
1. Keeping the jam-filled jars with jam upside down helps in the preservation process. It creates a vacuum and that's an old French trick. 2. 1 servings = 1 Teaspoon. 3 jars approx. 70 servings.