Blue hostas have a bluish crystaline waxy coating on their leaves, giving them a silvery-green or blue appearance. These crystals reflect the light causing the blue color. This coating, often called a "glaucous bloom," can fade as the season progresses, revealing the underlying green color. Some hosta varieties retain their blue hue longer than others. To retain the blue color most blue hostas should not be grown in full sun.  Some hostas also have this wax on their underside.