In a small sauce pot, combine blueberries, lemon juice and 1/3 cup of sugar. Cook, covered, on medium low heat for about 15 minutes. Stir occasionally. The blueberries should be fully cooked and mushy.
Run the blueberry mixture through the strainer to get the juice out. Press the blueberries into the strainer to get all the juice (you can use the leftover blueberries in yogurt, pancakes or waffles.)
Return the blueberry juice to the sauce pot and add the remaining sugar, cornstarch and POM juice while constantly whisking.
Bring the mixture to boil on medium high and lower the heat to medium. Cook the mixture until thickens, whisking frequently. Cook for about 10-15 minutes.