The blue-sky world is a Tracker slang term used to denote the overground or surface world. The majority of Trackers spend all their lives under the earthshield and never see the overground, the blue sky or the Sun on anything other than vid screens. The First Family have promised that the Federation will one day reclaim all of the overground and the people of the Federation will rightfully take their place as masters of the land.
According to the doctrine of the Federation, the blue-sky world will supposedly be opened up to widespread settlement in AD 3000, but that plan is currently running decades behind schedule. Radiation levels on the surface are still too strong for the average Tracker to survive more than a few years without protection, and psychologically most of the population is not prepared for the stress of living on the overground after spending decades in close quarters below ground.