Wash blackberries under running fresh water and dry them well.
Put blackberries into a large saucepan on a medium heat and cook for 20/30 minutsp.
Stir berries with a wooden spoon and try to crush them as much as you can to obtain a good mix of juice and pulp.
Turn off the fire and leave cool down for few minutes.
Take a potato masher (with tiny holes is perfect) and little by little, crush the mixture. Try to remove the most of seeds. Take the mixture of juice and pulp, weigh it and put it back into the saucepan.
Add the sugar to the mixture: for example if you have 4 cups of mixture (2 Lb. /1kg), adds 2.5 cups of sugar (1 Lb. / 0,5 kg).
Place the saucepan on a high heat, bringing the fruit mixture to a full rolling boil (you see bubbles). Then reduce to medium heat and cook for about 60 minutes, sometimes stirring with the wooden spoon, until the mixture become thick.
In the meantime, wash the jars, lids and bands (brand new or old ones) in hot water. Rinse well.
Sterilize them following this process.Take a large pot and cover the bottom with a towel. Put the jars and water inside the pot and boil for 10 minutes. Then, turn off the burner and leave cool down. Carefully remove the jars from the pot, shake out the excess water, and transfer upside down to a clean kitchen towel.
Once the jam is ready, pour it still warm into warm jars (they must not be cold to avoid to break. In case of need, warm them a little into the oven before the use). Close the jars and set them upright to rest for about 12 hours before to use it.