White Balance:

The White Balance control overrides the portion of the flare suppression algorithm which insures that whites in the foreground have an equal amount of red, green and blue.  It can be used to make foreground whites look warmer or cooler with minimal affect on most other colors.

When using a blue screen, the flare suppression algorithms can cause blond hair to look white around the edges.  Adjusting the White Balance control, so that whites in the foreground are warmer, can restore a more natural color to the hair without affecting other colors.

A White Balance adjustment differs from adjusting the Blue Level control.  White Balance is an adjustment in the blue content of white subjects.  Altering the Blue Level control  will affect all colors.