Place the blueberries, sugar, lemon juice and zest (if using) in a deep sided saucepan over a medium heat.
Stirring occasionally, heat the mixture until the sugar dissolves and the blueberries begin to release their juices.
Allow the mixture to boil and continue cooking until it reaches 220°F (105C) on an instant read thermometer.
Remove from the heat and pour into a large shallow heat proof dish to cool quickly. Or pour directly into a bowl or jar. Allow to cool completely and store in the refrigerator.