Opaque glass has been around since 16th century Egypt, and probably goes even further back to the Chinese in the 2nd century.  "Opaque" by definition means "not transparent or translucent; impenetrable to light; not allowing light to pass through."  Opaque glass can come in many colors - blue, black, pink and green (jade).  These days you'll also hear it referred to as "blue milk glass," "pink milk glass," etc.