They wouldn't be the first crew to fail to see a rate of descent until too late, if that did indeed happen. I can say that it was very black that night, with zero moon illumination, and little starlight. Beyond that, I can't speculate.
I must say one thing in defense of the accident investigator quoted, though. From reading the story, the speculation seems to be the reporter's, as usual. His oh so insightful questions and remarks seem fairly typical of reporters in general, and TV reporters especially. The statement that the S76 is the civil version of the Blackhawk is laughable. There is no quote from the investigator indicating any speculation, and in fact it is stated that the team refused to speculate, as is normal.