Black ball connector meaning the beer out connector? If so, a spray bottle with hot water and some oxyclean has worked for me in the past. Turn the keg upside down, and spray where the post and the ball connector meet. Let it soak and try to remove. If that doesn't work, unscrew the top of the ball connector and spray into the top of it. It may take a little while for the oxy to go to work, but it should come off.
Knucklehead Brewery, Est. 2007
Always do sober what you do drunk. That will teach you to keep your mouth shut. -Ernest Hemingway