Blueberries - I was told that Supermercado Amigo in Naco (Av Gustavo Mej?a Ricart, between Tiradentes and Ortega y Gasset) has sold them, fresh and/or frozen. They cost a pretty penny - can't remember how much but the image it evoked was that each punnet had its own seat on the plane.
I made a quick visit to this E.A.T. place when they first opened and did not find the people who worked there very welcoming, a feeling I've rarely experienced in all my years in the DR.
Anyway, the place had only just opened and was not fully stocked yet so I'll give them the benefit of the doubt. They actually had a pretty good range of imported international gourmet foods and the prices weren't completely heart-attack inducing. e.g. I was pleasantly surprised to see Lemon Curd (not gourmet, I know, but yummmmmmyyyy!).
It looked like they were working on a fresh produce section so this is another place where you might find things like blueberries.
Another good place to check out is the Italian deli on 30 de Marzo, between Parque Independencia and the National Palace. The staff there are friendly and helpful, the selection is great and the prices are very reasonable.