Place the blueberries in a small saucepan with the water and bring to the boil over a medium/low heat. Simmer for around 5 minutes until all the blueberries have burst.
Remove from the heat and pour through a fine mesh sieve or (idealla nut milk bag and leave to drain through. Once cool enough to handle, squeeze the bag to make sure all the juice comes through but the pulp stays behind. Return the blueberry juice to the pan.
Add the sugar and place back on the heat. Bring to a simmer again and stir constantly to make sure the sugar dissolves. Simmer for a few minutes and then remove from the heat and leave to cool.