Without all the attempts at writing poetry, the narrator saw gray clouds in the sky and the landscape in the distance. The rest of the underlined phrase is his/her description of the landscape.
We saw large swathes of silver-gray cloud hanging on the sky and underneath a stream of stark, metallic blue landscape in the distance.
I could not imagine what kind of landscape the narrator saw.
I do not understand what kind of landscape he saw. A stream of something that is stark, metalic blue color. What is the meaning of stark landscape? Can we say that a stream of mountain?
not a teacher
Perhaps it is the use of "stream" here that is also confusing for you. As you suggest, we don't talk about a "stream of mountains". In this case, the landscape probably does not have any mountains or other obvious features and is "desolate", as in one of the definitions in Mike's link. If the distant landscape is desolate and flat, or made up of low rolling hills, the description "a stream of stark landscape in the distance" is slightly unusual as a phrase, but it would be easily understood.
I wonder whether the metallic blue contrast to the stark landscapes. When anyone refered to the metallic blue car, I can feel the bright color, but the stark landscape is so desolated.