Add all ingredients to a saucepan and bring to a boil, then reduce the heat to a simmer (low heat) and let cook for 20 minutes.
Remove from the heat and use a wooden spoon or similar to crush the blueberries in the pot, really getting as much juice out as possible!
Pour the contents of the saucepan through a sieve (sitting above a wide bowl). When cool, pour the liquid into a jar/container and store in the refrigerator for up to 2 weeks.