Wash the blackberries thoroughly and place them in a large saucepan.
Add granulated sugar to the blackberries and stir gently to combine.
Zest the lemon and squeeze out its juice. Add both the zest and juice to the saucepan.
Over medium heat, bring the mixture to a boil while stirring frequently.
Once it starts boiling, reduce the heat and let it simmer for about 30-40 minutes. Mash the blackberries occasionally with a potato masher to break them down.
To test the consistency, place a small amount of jam on a chilled plate and run your finger through it. If it wrinkles and doesn't flow back together, it's ready.
Remove the saucepan from heat and stir in the vanilla extract.
Allow the jam to cool slightly, then strain it through a fine-mesh strainer to remove any remaining seeds.
Pour the seedless blackberry jam into sterilized jars, leaving a little space at the top.
Seal the jars with lids and let them cool to room temperature before storing them in the refrigerator.