My 4 year old has a lump on neck behind ear it is a blackish blue color. her doctor does not know what it is sending her to dermatoligst. it has grown?

First thought benign: My first thought on this description is a benign blue nevus, an distinctive round to oval spot that firm, grey blue to blue black & can occur anywhere on the body. They usually have a well defined border & appear gradually in a patient. It could be something else & the dermatologist can decide if special tests are needed to decide what it is.