Add frozen blackberries and agave to a pot over medium heat
Heat the blackberries until they start releasing juice and then bring them to a boil.
While the blackberries are boiling, stir occasionally until the berries begin to break down
Stir in chia seeds and vanilla and continue to cook the mixture for another 5 minutes
Remove from heat and allow jam to cool
Transfer jam to an airtight container and let it sit in the fridge overnight to finish thickening up.