Mix the blueberries with the sugar and lemon juice in the saucepan and put over medium heat.
Stir as you heat to move the sugar around. The sugar will first dissolve into a syrup and then begin to foam as water evaporates (10 minutes).
Continue cooking and stirring. The temperature should be around 105 C / 220 F.
After 20 minutes, the syrup should begin to thicken.
After 25-30 minutes the syrup should be thick enough to coat a spoon and has a consistency almost as thick as honey. See recipe post for tips on checking if it's the right thickness.