Is that as black as it looks in the pic? Also, did this happen all at once? Usually, clutches will start to slip before the vehicle won't move. If that isn't what happened and it did happen all at once, it does sound like a pressure issue.
I reread your original post. If the fluid was that black and you were having issues as described, the new fluid could have finished off the clutch friction plates. Once they get extremely dirty and wear, the new fluid actually will clean the dirt off the clutch plates and cause them to slip. However, before I would say that is the issue, make sure the linkage is properly connected. If it is and the gear indicator is accurate, I would check line pressure.
Have a helper shift through the gears while you watch the shift cable on the trans. See attached pics (1 and 2). There is a clip that could have broken that holds the cable in place. If it broke or fell off, the cable won't shift the trans properly, so check that too.
The remaining pics explain how to test line pressure. You will need a gauge and scanner to perform the test. The last pic includes the line pressure specifications.
I hope this helps.
See pics below.
Images (Click to enlarge)
Friday, April 16th, 2021 AT 8:10 PM