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Sounds like the overfill didnt do to much damage, shouldnt have actually, normally it would just shoot it out the rear or another seal thats not tight, in your case you let it out before it found its way out, the black fluid was probly becauseyour tranny is dirty, and the late shifting is probly because you have that dirt in your valve body , additionaly fluid that is black is not nearly as slipery as ill say which would mean it has to heat up before anything can move, thus the ball's inside the valve body cant move till this point, if i was you id pull the pan, take the valve body out which has a billion screws holding it in, all of them are 10mm, drop that and all that excess black fluid will fall out when the valve body is dropped, replace the valve body, put pan back on fill up drive for around 50 miles drain fluid and fill it back up, the color should go back to normall, if it doesnt youre tranny discs are about to take a crap on you and that means a rebuild.