Low blue sys hyd level is a bog easy fix (if it is just a routine top up). But.. it should NOT be something that should be found during turn around. Hyd levels are checked overnight and should not need topping up for quite some time in a good system . It should not take 5.5 hours either. I assume it was a 320. The blue sys can be a bit tricky getting a head of pressure but 5 hours tricky, no way under normal circumstances. The real circumstance may have been extraordinary but having to add some skydrol should take 5 minutes. So their claim maybe correct but the reason given , in isolation, is BS imho. Now if they really meant low sys pressure (not level) , that would be different again meaning a pump failure and could be described as extraordinary.