Did you try some Lucas transmission fix with your fluid change? It is amazing what this stuff does. It is like the Seafom of transmissions.
The dark color of the fluid would indicate lots of clutch and band friction material in the fluid. The inability of the transmission to shift out of 1st gear without "letting up on the throttle" would indicate a 1-2 clutch band issue. I'd try the Lucas stuff before spending more money and time on it. The bottle says use 12oz for smaller cars and two bottles for bigger cars. I use about 20oz in the transmission and 4oz in the power sterring system (which you should also flush out).
Hope this helps.
