Combine the blueberries and sugar in a heavy saucepan. Stir gently to mix, then put the vanilla bean into the center and let stand for about an hour.
Add the water and lemon juice to the blueberry mixture and bring to a boil over high heat, stirring occasionally. Reduce the heat slightly and simmer, skimming off any foam that comes to the surface, until the mixture starts to thicken.
Remove from the heat and strain through a fine sieve. Cool to room temperature and refrigerate tightly covered until ready to use.