OK. When you bulk load, both ends of the film are exposed to light. Both ends will come out black if you have processed the roll anywhere near correctly.
If the ends are black, it indicates that the film is not being transported correctly in the camera. You can check this in most 35mm cameras by loading the film and then, without pressing the rewind button, gently turning the rewind knob until you feel resistance. Then, when you advance the film, the rewind knob should turn. If it doesn't, it's a bad load [film tongue not caught by the take-up spool] or a defective mechanism. You should make this check a part of your camera technique for each and every roll you shoot.
Also, if the ends are black, another [remote] possibility is that you're mixing up the film cassettes and processing film which has not been used. You keep this from happening by rewinding all the film, including the leader, back into the cassette when you finish exposing a roll of film.
If the ends of the film are not black, it's your development process. We can trouble-shoot that for you if it's the problem.