Blue Forest

The Blauwe Bos is a forest and nature reserve in the east of Friesland that covers more than 300 hectares. Location: in an otherwise agricultural environment in the triangle of Haulerwijk, Haule and Waskemeer. It is owned by the state and managed by the Forestry Commission.
According to tradition, the Blue Forest owes its name to the bluish color that the forest gets from the trees that grow there. Another explanation of the name is based on the fact that the word "blue" used to be widely used in the sense of "bad, inadequate". That would fit in with the fact that the forests are planted on acidic peat and heath soils.

Wikipedia