If the fluid is already black, by all means it should be changed.
However, once trans fluid gets to this stage, it is possible that a fluid change at this point is too little, too late. Think of it as a Hail Mary Pass that might save the day.
Yes, the Toyota maintenance schedule claims that the trans fluid is “lifetime” or “long-life” fluid. However, only people who believe in The Easter Bunny should believe fables like that.
Trans fluid should be changed every 30k miles or 3 years (whichever comes first), even if the car mfr does not list this service. When a trans is not serviced on that schedule, trans failure can take place any time after ~90k miles, and is pretty much of a sure thing after ~120k miles.
By that measure, the OP’s discolored trans fluid seems to be very premature, but…if the vehicle was ever used for towing, or if it was used for a lot of mountain driving, or if the engine ever overheated, it is possible that these factors led to such early symptoms.
Yes, change the fluid and the filter–if it has a trans filter.
Make sure that your mechanic uses ONLY genuine Toyota trans fluid, rather than a generic fluid that is supposedly OK in all imported cars. (It isn’t)
Then, begin a Novena for the continued health of the transmission, and also resolve to always have your transmission serviced on the schedule that I noted above.
Sincere good luck with the transmission!