It's not dirty. In fact, blueberries with a white cast are actually fresher.
The white haze is due to a natural wax layer that protects the blueberries against external influences. The layer ensures that the blueberries remain fresh and juicy. It is therefore a good sign if the blueberries in the supermarket have a white layer.
Not only blueberries have a protective layer. Grapes sometimes have this too, but the lighter color makes it less noticeable.
