In a large bowl, wash your fruit and remove any small branches or leaves. Drain well.

In a large pot, pour your blackberries, sugar, and lemon. Stir the mixture to coat all the fruit with sugar.

Stir the mixture to coat all the fruit with sugar. Cook covered on medium-low for about 10 minutes, reduce heat to low, and cook another 30 minutes. Stir occasionally. You can test to see if the jam is thickening by using a very cold plate from the freezer and testing a small amount as you cook.

In the meantime, clean your jars with hot water and soap or run them through the dishwasher.

Allow the jam to cool down completely before transferring into the jars.

Pour the jam and store it in the freezer for up to a year or in the fridge for a couple of weeks.