A Blueberry is an entity that is significantly larger and heavier than a normal human. While blueberries as fruit are natural, blueberries as an entity can only be achieved by ingesting the Three-Course Dinner Chewing Gum. The only confirmed specimens are 20 Oompa-Loompas and Violet Beauregarde.
When first going through the transformation, the victim's nose slowly turns blue, and it spreads across their face. The color change then speeds up, affecting the victim's hair and neck before spreading to the rest of their body. When the victim had turned completely blue, the swelling portion starts, starting with the victim's hips and lower body and turning the victim's garments blue along with gurgling. The butt instantly doubles in size as the stomach starts increasing in size, with the inflation speeding up. The whole body then starts swelling, doubling the size of the victim's cheeks and slowly sucking the victim's head, feet, and arms into their body, all the while increasing in size. The transformation stops abruptly, but not before the user reaches an absurd height and becomes completely round, save for sunken indentions at the hands, feet, and head. The victim is left unable to move their body; their only form of travel is being rolled by an external force. In this stage the blueberry is "ripe."
A blueberry transformation is unfortunately permanent, as they would have their skin, eye, and hair color changed to a permanent shade of blue. However, depending on the size and/or how long one has been a blueberry, the juice can be removed from the body through a process called juicing, allowing blueberries to return to their original, human size.
There are advantages blueberries have compared to humans. Most notably, due to changes in their bone composition, juiced blueberries gain inhumane flexibility, allowing them to bend any part of their body without injury. This can be seen as Violet is leaving the factory performing acrobatics that normal humans could not do. Furthermore, blueberries have strong resilience, as Violet was able to swell to an enormous size without exploding and was rolled around the Inventing Room without any major problems. Finally, the juice itself may have some special effects in making materials more flexible, as Violet's clothes were able to accommodate to her new size without any sign of ripping.
Twenty Oompa-Loompas have undergone the transformation while testing the gum, however, none of them show up on-screen.
Probably the most famous blueberry is Violet Beauregarde, who stubbornly ignored Willy Wonka's warning and chose to chew the gum, transforming her into a blueberry. Luckily, she was juiced, thus retaining some of her human traits.