the Air Force Chief (or Army or Naval Chief of Operations) run their respective services.
The Joint Chief works for the President and acts as the conduit between the civilian leadership and the service Chiefs.
No Black Man or Woman has ever led a service and I believe only one person of color has done so - General Shinseki, Army COS
Gen. Powell slid over from his job in the White House as National Security Advisor - He already had a working relationship with Bush - and that's all Bush was looking for in his JCS.
Usually (but not always) one of the sitting chiefs slides in to the JCS slot for 2 years and then retires