Glass Blocks Invisible behind carpet

Glass blocks (not panes) are invisible from the front when viewed directly behind a carpet. Does not APPEAR to matter whether there is a block underneath the glass block or not, also color of carpet seems to matter. Discovered this when outside house, had carpet placed in front, I didn't see a glass block. Going back inside and looking outside where there is no carpet in front of it, I saw the glass block just fine. Rain does not make a difference either (I don't think, tested when and when not raining).