In a medium saucepan, boil 1/2 cup of the port wine and the sugar for 3 minutes to cook off some of the alcohol.
In the meantime, in a small bowl, whisk together the remaining 1/4 cup port, vanilla, and cornstarch. Add it to the pot and cook until clear, about 3 more minutes. Add 1 pint of the blueberries and continue to cook until the sauce thickens and the blueberries soften. Let cool and add the other blueberries. Serve with a healthy dollop of sour cream.