hi gary,
well most certainly dark fluid is not a good sign.
but for starters i'd change out the fluid and filter and clean the sump screen if there is one at the base of the trans housing.

many things can cause no movement
- low oil level (which you already discounted this one)
- oil pump drive
- torque converter system
- bad clutch packs
- inching valve system
- clogged oil flow
- faulty control valve

i guess i could keep naming things but these are for starters.
i'd also get a guage and check pressures at the control valve, torque converter pressure and clutch pack pressure. converter pressure should run around 43-100psi @ 1500rpm, and clutch pressure should be 149-199psi. There should be 2 ports on top of the tranny, one ON the control valve for clutch pressure and one just behind the control valve on the case just to the left side rear of the control valve for converter pressure.