People kept telling me they were the same... so I looked and it appears they are not quiet the same.
Bilberry (Vaccinium myrtillus L.) is a small shrub growing on hilly heaths and under-bush throughout Central and Northern Europe. It grows wild in the Northern European forests and, differently from blueberry, that is a cultivated hybrid of three native American species (V.corymbosum, V. ashei and V. angustifolium), its fruits are not produced in clusters, but only as single, or rarely, twin fruit. Additionally differences between these two berries are the fruit size, as bilberries are smaller and darker than blueberries, and the pulp colour.
Bilberries pulp is blue, that of blueberries is greenish. While bilberries are harder, less juicy and easier to transport than blueberries.
Finally bilberries and blueberries have a different phytochemical profile and anthocyanins content of bilberry fresh fruits is almost 4 times higher in bilberries than in blueberries.http://www.mirtoselect.info/public/bilberry_blueberry.asp