Note: this step requires that you have a digital scale and that you feel comfortable with basic maths; if not, you can skip this step and follow the instructions in the questions & troubleshooting section below.
Prepare your workplace: place a digital scale next to the stovetop. We will be weighing the saucepan during the cooking process, so place a trivet on top of the scale (or a folded kitchen towel) to protect it from scalding.
Calculate the saucepan+blueberries+sugar weight after cooking: in a medium saucepan, put the blueberries (450 g; 15.9 oz) and the sugar (170 g; 6 oz) and weigh the filled saucepan. We want to boil out 100 g (3.5 oz) of water from the blueberries. So, from the “saucepan+blueberries+sugar weight”, subtract 100 g (3.5 oz) and take note of the result; this is the target weight of the saucepan and its contents after cooking.
Warm the blueberries with the sugar: place the saucepan over medium heat, and warm the blueberries, often stirring with a rubber spatula until all the sugar dissolves (this is at 65° C / 149° F if you have a thermometer).
Increase the heat to medium-high and bring to a boil, stirring often. Boil the blueberries, weighing the saucepan as needed, until it reaches the target weight (this should take about 5 minutes).
Let it to cool down in the saucepan until lukewarm.
If using an immersion blender to blend the blueberries in step 2, scrape the blueberries into a large bowl. If using a regular blender, prefer to scrape the blueberries directly into the blender’s jug and store it in the refrigerator until cold. We need to chill the blueberries in the same bowl/jug we will use in step 2 to blend them to minimise the loss of blueberries from transferring from one bowl to another.
Chill until completely cold: cover the bowl/jug and refrigerate for at least 8 hours, or until completely cold (approx. 8° C / 46° F / when it feels fridge-cold to the touch); and up to 3 days.
If you have a thermometer, read a faster way to chill the blueberries in the questions & troubleshooting section below.