In a small heavy saucepan, place blueberries, 1/2 cup of the water and granulated sugar.
Cook and stir on medium heat until all sugar is dissolved.
Bring mixture to a full boil, then turn down to simmer.
While mixture is simmering, mix remaining 1/4 cup water with the cornstarch, stirring well. Add cornstarch mixture to blueberries, stirring while adding, until it is all incorporated.
Continue to simmer for 15 minutes, or until syrup reaches desired thickness.
Remove from heat and add lemon juice. Cool. After the syrup has completely cooled, it can be added to buttercream. This will keep in an airtight container in the refrigerator for up to 2 weeks.