Hi another expert here to assist you with this issue .
First information Dave gave you is correct .but with my experience with this transmission you would save a lot of cash if you just have it rebuilt or take a chance with a used unit .
Having black transmission fluid is a good sign of internal damage to the clutch plates . the small clutch parts have mixed in with the transmission fluid and go into all the small working parts of the valve body .
This cause particles to get into the valves solenoids and spring balls causing all sorts of problems .
Lets say you back flush the system remove the valve body clean it out and then put all back together there is always that small amount of dirt that will cause you problems in the very near future ,
Ss i suggest you replace the transmission with a new converter .
Been there and done it many times as requested by the customer only to come to my conclusion of having to replace the transmission or rebuild with a new converter .
The converter will hold a lot of dirt even after flushing and draining .
Need further assistance with this issue then please get back to me .