In a large saucepan, combine the blackberries, sugar, and lemon juice. Make sure there is enough headspace to allow for foaming.

Turn the heat to medium-low and simmer the mixture, stirring regularly to keep the bubbles down. It should take about 20-30 minutes for the jam to reach the gel stage, depending on the sugar amount. (More sugar will cause the jam to gel faster and yield a higher amount.)

To test if the jam has reached the gel stage, place a plate in the freezer and drop a small amount of jam on it. If it thickens, it's ready.

Once the jam is ready, pour it into prepared canning jars, leaving 1/4 inch of headspace. You can store the jars in the refrigerator or process them in a water bath canner for 10 minutes.