Add the frozen blueberries, 10g honey, 1 teaspoon of lemon juice and the vanilla extract to a small saucepan. Cook over a low-to-medium heat for approximately 5 minutes, stirring regularly, until the blueberries have softened and released their juices. Mash the blueberries with a fork, then stir through the chia seeds. Remove the pan from the heat and leave the jam to thicken for at least 15 minutes.