Wash the blueberries and blend them in a blender.
Mix the blueberry puree with the water in a pot on the stove and boil. Let it boil on low heat for about 3-4 min.
Remove it from the stove. Strain the juice to get rid of the seeds and skins of the fruits.
Put the syrup back in the pot and boil 15 min. Add the lemon juice and remove from the stove after 2-3 min.
Distribute the cooled syrup in bottles and store in the fridge.