A firm blueberry jam is denser and less spreadable, whereas a soft jam is more fluid and spreadable. To make a firm jam, you need to cook it for a more extended period to allow the natural pectin present in the fresh fruit to thicken the jam. Alternatively, you can add some pectin or incorporate high-pectin fruits such as apples to assist in the jam's setting process. Conversely, for a softer jam, you must cook it for a shorter time to obtain a more fluid texture. You can add some liquid, such as water or fresh lemon juice, to keep the texture of the jam soft.