Bluing whites?

Have you ever heard your mum or grandma mention bluing her whites to make them whiter? How do you suppose that worked? One would think that to make things whiter you would wan to add a whitener (like bleach) not a blue... well I've figured it out (thanks to Wikipedia).

"Bluing, laundry blue, or washing blue is a household product used to improve the appearance of textiles, especially white fabrics. Used during laundering, it adds a trace of blue dye (often synthetic ultramarine, sometimes Prussian blue) to the fabric.

White fabrics acquire a slight colour cast after use (usually grey or yellow), because they can never be cleaned perfectly. Adding a trace of blue color to the slightly off-white color of these fabrics makes them appear whiter. Laundry detergents may also use fluorescing agents to similar effect.

On the same principle, bluing is sometimes used by white-haired people in a blue rinse.
Bluing has other miscellaneous household uses, including as an ingredient in rock crystal "gardens" (whereby a porous item is placed in a salt solution, the solution then precipitating out as crystals), and to improve the appearance of swimming-pool water.

Bluing was invented by William Hawkins Errington (1802-1877) which he manufactured as Ricketts Blue in the Blackfriars Road Southwark, and was popular up until the mid 20th century. Since then, it has largely been replaced by the use of bleach for its primary purpose.
Bluing is usually sold in liquid form, but may also be a solid. Solid bluing is sometimes used by hoodoo doctors to provide the blue color needed for 'mojo hands' without having to use the toxic compound copper sulphate."

http://en.wikipedia.org/wiki/Bluing_(fabric)