Does the Dutch word you are translating as "black" actually correspond to the same range of colors as the English word "black"? I am assuming this is Vaccinium myrtillus (European species) and not Vaccinium cyanococcus (American species), although Google image searches result in a similar color range for those two, at least on my monitor.

Another example, to confuse things further, is that in Scots, they are "blaeberries", and "blae" is a separate word from "blue" ("Bluish in colour, of a dark colour between blue and grey, livid." — http://www.dsl.ac.uk/entry/snd/blae).

Presumably our ideas of what "blue" encompasses come from what objects already get referred to as blue, so, circularly, we're likely to describe them as blue ourselves?