Put all ingredients in small saucepan and bring to a boil. When it has cooked for about 10 minutes, mash the blueberries, stir and then strain through a sieve or strainer and using a spoon keep pressing on the berries until you have all juice pressed through the strainer. If this is still a little thin, continue cooking until it has thickened. Squirt a little on top of your mustard and you can serve the rest of the sauce in small dish for those who want a little more blueberry with their mustard.