Sultan - I disagree, I have encountered full on ground resonance in a Wessex and the early stages in Sea King - there is enough time to do something about it but you need to have been trained to recognise and deal with it.
As soon as they landed and experienced the 'padding' that precedes GR, they should have lifted to the hover - if the oscillations stop, that is when you consider a running landing or a low hover throttle chop.
How much GR awareness is there amongst the Blackhawk users? If it is a very unusual occurrence on that aircraft then it is understandable why they didn't recognise it and act quicker.
I flew the Blackhawk S-70 for four years. Unlike with the Wessex, I have no recollection of the type having a ground resonance tendency. I think Sikorsky had already learned their lesson.