I am assuming that when you replaced the TCC solenoid, you fixed the "TCC is always locked up" problem and that you changed the fluid at that point. Regardless of why the black goo is there, if there is A LOT of black goo you're in toruble. If it is just a little bit, that is normal as noted by HOTTOGO.
Do you necessarily need a rebuild? All depends on the amount of power you are putting to the trans and how well you need it to hold up. What are your goals with the car?