Sounds like the black box may be a 5V voltage regulator and current-limiter. There is a chance it still works, and only the cells are dead. You might try disconnecting the cells from the black box and providing a standard 9 volt input to it, while measuring its output voltage. This is only a good idea if you are used to dealing with electronics, but you sound as if you are. The upstream diode is probably there in case some amateur reverse-polarises the input, which would fry the regulator if the diode wasn't there.
