Constantly sick and I have no idea why..!?

I've been sick 6 times since October 2010 with chronic coughing and it seems like just a really really really bad cold! I've been to the doctor many times and he has diagnosed me with asthma and bronchitis, but it keeps coming back!! I have an inhaler and I do it everyday, twice a day. It will work for about three weeks then I'll just start getting sick again! I occasionally have a fever and I am always (since October) coughing up nasty mucous-y stuff. What really freaks me out is about 60% of the time there's this nasty black ink-like looking stuff in it. Not that much but its gross and I don't know what it is! Does anyone have any idea what might be going on?? Thanks!!