Blue tubes are usually just grey tubes that have been painted so they look blue, but if you were to very carefully chip the blue paint off, you would see that they're really just ordinary grey tubes underneath.
Blue tubes are sometimes used to hold up street lights.
[Dude man this is Larry and this is like absolutely true that I chipped a grey tooth on a blue tube once coming home from an ELO concert. Fuckin' unreal. - Larry]