In a small saucepan combine blueberries and lemon juice; bring to a simmer over medium heat and cook, stirring constantly and gently smashing the blueberries smooth, until thickened into a deep, dark blueberry reduction, about 10 minutes. Remove from heat. Place a fine-mesh strainer over a small bowl and push the puree though, leaving any skin/large chunks behind. Allow mixture to cool completely before using. When ready to use, follow instructions above.