According to Wikipedia, milk glasses were first made in Venice in the 16th century, colors include blue, pink, yellow, brown, black, and white. 19th-century glass makers called milky white opaque glass "opal glass". The name milk glass is relatively recent.
These days, milk glass still remains a popular collectible for around the home. Although it is not as expensive as porcelain, I think it makes a beautiful alternative for all sorts of decorations and decor items.