The cerebellum is a part of the hind brain, located at the base of the skull (purple side lobes in picture). (newscientist.com)
I think it helps lend a little more blue color to the final layer, but if you use just blueberries the color is still fine — just a little more on the purple side. (detoxinista.com)
I would say it definitely came out lighter on the purple side but I don't know why. (cupcakendreams.com)