Gray represents depression and melancholy, which can better reflect the despair in your poems than black.
Gray seems to be on the edge of hell, boundless. Although black can also reflect the sad mood, it is more mysterious and attractive, which is not in line with your poem.
Gray can be used to describe dust, but black can't, which echoes "Phoenix Mountain" and "Stone Gas".
Black gives people the feeling of loneliness, helplessness and deep fear, but what you want to express is the desolate and barren artistic conception, which is inconsistent with it.