Blueberries are actually not blue, but a purplish black. The bloom is what makes them look blue. Modern cultivars have been selected to have lots of bloom and look blue. Many wild blueberries will have little or no bloom, and even within a closely related population, may vary quite a bit from plant to plant.

Ditto kiwinut about how not all bluberries have bloom...the ones that don't are quite dark. However, if you were noticing variation between berries on the same plant, it could just be from the leaves rubbing on the berries. I've seen berries (they were still attached to the bush) where it was clear that's what happened.