In a small sauce pot, combine blueberries, lemon juice and 1/3 cup of white granulated sugar. Cook, covered, over medium-low heat for about 15 minutes. Stir occasionally. The blueberries should be fully cooked and mushy.(Watch the pot, if the heat is too high, it will boil over.)
Run the blueberry mixture through the fine mesh strainer to get the juice out. Press the blueberries into the strainer to get all the remaining juice. (Tip: You can use the leftover blueberries in yogurt, pancakes or waffles.)
Return the blueberry juice to the sauce pot and whisk in the remaining sugar and POM juice. Combine cornstarch with a splash of cold juice or water, mix well. Pour in the cornstarch slushy while constantly whisking.
Cook until thickened. Bring the mixture to a gentle simmer over medium heat and lower the heat to medium-low. Keep slowly whisking until the syrup thickens. Cook for about 5-7 more minutes, stirring from time to time.
Cool completely before serving.
Notes
Storing: Transfer cooled blueberry syrup into a glass jar with an airtight lid and refrigerate for up to 2 weeks. To get some out, remember that it’s best to pour some out of the jar rather than using a spoon.
Freezing: Transfer cooled blueberry syrup into a freezer-safe container that can be closed airtight. It can be a plastic freezer food storage container or a freezer zip-top bag. You can store all of the syrup at once or portion it into smaller batches. Get as much air out as you can, label, and freeze for up to 3 months. Thaw slowly, overnight, in the refrigerator.