The bilberry (Vaccinium myrtillus) is a member of the heather family and here in Dorset it prefers higher areas of heathland around Puddletown Forest and Thorncombe Wood. Elsewhere in Britain it is a common plant of the upper moorlands in the north country.
The pink flowers of spring produce dark, almost black, berries in mid-summer which are useful food for a number of creatures and are favoured by us humans too! They are closely related to blueberries which are now common in British supermarkets. The bilberry is the famous huckleberry of north America.
It has a number of local names across the country including whortleberry here in the south but bilberry is the one favoured by the reference books.
The beneficial aspects of bilberry juice include a supposed improvement in night vision and the Wikipedia entry for bilberry has an interesting note about wartime RAF pilots eating bilberry jam! You can read it here: https://en.wikipedia.org/?title=Bilberry