Air at 50m is still doable, at that depth the PP02 is 1.26, it is not strange that experienced divers do it, they know what they are doing, knows their SAC rate and the MOD of their mix, they dive with people who is equal experienced and know what to do if something happens, they have a thoroughly dive plan and sticks to it, they have bailout bottles waiting for them on the deco stops, they have been in bad situations before and can keep their head cool when the FUBAR hits the fan. That said, no matter how experienced you are there will always be a risk, all you can do is minimize it. Most of the divers who die on dives like this is the inexperienced diver without the necessary qualifications to do the dive, who jumps into the water with a equal inexperienced buddy, with the dive plan "Descend to 50m swim around for a bit, then head up again". I haven't dived the blue hole, but from the pictures and videos it doesn't seem any more dangerous than many other places with clear and warm water and superb visibility, But I think dive sites like this attracts a lot of divers, and therefore also attracts a fair share of inexperienced and reckless divers that results in higher then normal accident rates.
You need to see the videos, almost all mention experienced divers, that dosen't mean good planners, all of the people that die, did not had deco bottles waithing for them, nor pony bottles attach