Simply put...they all use metaphors to explore or describe change. In the poem "Blackberrying", Plath seems to be establishing a connection with the blackberries. The point of transition seems to be when she follows (sheeplike) the path to the ocean.The change seems to occur at the point where the blackberries end. Here, she is confronted with wind and emptiness. It is a human world, but it is one devoid of human emotion.(The world connected to the blackberries, on the other hand, is almost spiritual. Plath talks about the experience of the flies amidst the overripe berries as being almost spiritual in its sweet excess: "The honey-feast of the berries has stunned them; they believe in heaven.")
In direct contrast, once the protective berries are left, the persona is confronted with the harsh elements of nature. The wind slaps her, but she does not cry. We don't see her meet the sea as much as emptiness--the orange rock face looks out onto "nothing but a great space of white and pewter lights." If the berries "loved" her and became joined, unasked, in "sisterhood", then the scene she arrives to is the opposite, a "din of silversmiths beating an intractable metal."
The elements become her metaphor for change. The change in her perception is reflected in the change of scenery.