Blackberry jam is my favorite kind of jam. When I was a kid, I used to go and pick blackberries with my sister and my dad, and then we would bring the berries home, and my mom would make blackberry jam and Danish pancakes. My jam recipe is the same as my mom’s (and most other recipes), but with a little twist, as I’m adding vanilla and rum. This gives the jam a more grown-up flavor, which I really like.
I used gelling sugar (jam sugar) to make the jam in the photos of this recipe. Gelling sugar is granulated sugar with added pectin. Pectin is a naturally found in all fruits and berries, and when heated, this is what gives the jam its jelly-like texture. The pectin concentration varies within species with lemons and apples among the most pectin-rich fruits, and within the ripening cycle. Ripe fruit has less pectin and is thus more difficult to jam. Blackberries are high in pectin, so it is possible to make the jam with regular granulated sugar, but if in doubt, or if you prefer a really thick, jelly-like jam, add a tablespoon of lemon juice (it contains a lot of pectin). For the jam in the video below, I am using jam sugar, and it took a bit longer (about 35 minutes) until the jam had the texture I wanted.
You can check the viscosity of the jam along the way by scooping up a bit of the jam on a spoon and put it in the fridge for a few seconds, or you can use a candy thermometer and wait for the temperature of the jam to reach 220°F (105°C) which is the gel point, i.e. the temperature point, when the jam starts thickening. Depending on how you choose to sterilize the jars, the jam can last up to 1 year.