In adult life, red blood cell production takes place in the bone marrow; however, during normal development of the unborn - in the uterus - formation of red blood cells occurs in organs such as the liver, spleen and skin. Under certain circumstances such as in infection with the rubella virus or cytomegalovirus, red blood cell formation in the skin continues abnormally after birth giving the skin blotches that seem like blueberries on a muffin – blueberry muffin baby/rash.
Additionally, certain cancers and vessel abnormalities may also give the blueberry muffin baby appearance.
Mehta V, Balachandran C, Lonikar V: Blueberry muffin baby: A pictoral differential diagnosis. Dermatol Online J, 2008 14(2):8. Go to reference
Blueberry muffin appearance of neuroblastoma (a cancer) and a muffin