Full black water tank

Hi.. sound like the tank will no dump by opening the gate so the tank can dump. The cable from the gate runs down to the handle which is pulled to open the gate. This most of this cable is covered by the under belly...lubricant the cable from handle to we it goes into under belly. Problem maybe cable hiden in belly is dry, cable is kinked, or cable is disconnected. From gate. To get to most of the cable and gate you will need to drop belly. This is a major job. Camping world will do it or you can do it your self.. I had this problem on one of my trailers. Jim2007

Sounds to me like a potentially nasty job.....

For a one time thing it would be okay.
I'm guessing you have a valve problem and can't dump your tank the normal way?
I wouldn't want to do it on a regular basis.

why would you?
Is the grey tank full too?