Sterilize canning equipment. You can boil your supplies to make sure they are clean and ready to go.
Cook blackberries on high heat until boiling. Boil for 3 minutes.
Stir in lemon juice and sugar until boiling. Boil for 3 minutes.
Remove from heat and whisk for 3 minutes.
Pour jam into mason jars and place lids tightly.*
*Make sure to fill your mason jars up to the top and secure lids tightly or they will not "pop". If the jars do not seal correctly, you can still store in your refrigerator for later use!
**If you desire to use a hot water bath at the end to get the jars to "pop" that works great as well! Simply place jars in a boiling pot of water for 10 minutes. When you set them out to cool down afterwards, you should hear the "popping" sound as they seal!