My Boxer Has Black Areas On Her Skin, Which Turn Into Sores, Then The Skin Splits Open And Leaves A Raw Place. What Is Wrong?

Your dog needs to be examined by a veterinarian.  When the skin turns black this is call necrosis-or death of the skin.  Dead tissue often becomes secondarily infected as the cells releases toxins as they die harming the healthy tissue around it making it susceptible to normal bacterial flora.  A space the size of 2 hands is a large amount area.  I would be very concerned about this spreading.
Spider bites can cause large areas of necrosis and infection.  A severe skin infection can cause this as well.  Neoplasia (cancer) of the skin can also cause necrosis and again a secondary infection.
Your veterinarian will help determine the disease process affecting your dog, make sure there is no underlying condition and treat it appropriately-potentially with anti-biotics, anti-inflammatories-- both oral and topical medications.  Some wounds need to be debrided--the dead tissue is removed and then closed surgically.  This can speed healing and aid the treatment of infection.

This sounds like a skin infection, which certain breeds are prone to. I would take your dog to a veterinary Dr. They have antibiotics for dogs. It takes a while to clear a skin infection from a dog, so the earlier the better.