Drugs, all you say makes sense - what is difficult to understand is that they acknowledged the little islands out of hand, without really questioning why they hadn't seen them on radar before they overflew them. That should have been the first alarm bell that their route wasn't as safe as they had assumed.
If it had prompted them to check the radar more closely and expanded the scale, should the return of Blackrock have been visible even if it was partially obscured by the FMS WP marker?
Clearly they weren't expecting to have to manoeuvre, hence the FMS route guidance, but would you expect a slightly quicker response in the cockpit to a concerned call about terrain from the back?
What makes you think they hadn't seen them? I'm pretty sure they had seen them, they were an experienced and capable crew. They just thought the route was safe and that they were low lying islands.