Wash blueberries under running water, drain them and put them in a large pot. Add the grated zest of two lemons and their juice.
Cook the blueberries on low heat for about 30 minutes, until they start to simmer.
Add the sugar, stir to melt it and bring the blueberries to the boil again. Let them simmer on low heat for about 45 minutes. Check the thickness of your jam: drop some jam on a frozen saucer, if it sets, it’s ready.
Pour the jam into sterilized jars and close them tight. Put the jars in a large pot and cover with water: bring to a boil. Simmer for 20 minutes and then remove from the heat. Let the jars cool completely in the pot, then remove them from water. You can store them for several months in a dry, cool and dark place.