Internal Bleaching is a special technique used for removing stain and pigment when it originates from the inside of a single tooth (unlike stain that comes from food and drinks which are seen evenly deposited on all teeth). This is generally seen on a single incisor in the front of the mouth, or on a canine, and often correlates with a history of trauma to the tooth in the past. These are teeth which sometimes start out with a purple or even blue appearance. In some cases, root canal treatment is needed when the tooth has not already been treated with one. Patients often find Internal Bleaching to be an exciting process with results that seem to magically appear.
Internal vs External Bleaching
This video explains a bit about the difference between internal and external bleaching of teeth.