I think the atmosphere and mood that is created through those words is a suspenseful, tense, and ominous atmosphere. Dorris Lessing needed to use words like "dark" and "looming" in order to change the initial atmosphere of the story. The story starts out with a mother and her son on vacation. They are staying at some kind of beach resort. Beach resort and vacation. That immediately creates a relaxed and happy atmosphere for the reader.
Lessing changes the mood of the piece after Jerry goes out on his own to the wild beach. Once that happens, Lessing uses the word "dark" frequently.
He could see the hole. It was an irregular, dark gap; but he could not see deep into it.
She uses it to describe visibility through the water. She uses it to describe the hole itself. Lessing also uses words that are descriptors of large items -- "big, heavy, looming," etc. All of those word choices together alert the reader to the presence of danger despite the relaxed vacation atmosphere. Doing things that are dangerous create apprehension in people. There is an ominous atmosphere about those actions, because there is always the risk of failure. Lessing's word choices help cement the danger and ominous overtones in the reader's mind about what Jerry is going to attempt.