In a medium kettle, combine blueberries and water.

Bring to a boil and mash berries slightly with potato masher.

Continue boiling 5 minutes.

Drain well and puree in food mill or food processor.

Back in the kettle, combine berries, sugar, and lemon juice.

Bring to a boil over medium heat.

Reduce heat and boil gently, stirring often, until thickened and spreadable.

Ladle into clean jars and process for 10 minutes in boiling-water bath.