I wouldn’t do it either. When you have a used saddle that has absorbed many substances, some intended, some not so much, and the leather worn and slightly roughed in some places more then others? Nothing is going to absorb equally on those unequally worn surfaces and that could either enhance the saddle with a nice patina of varying shades or come out just plain old splotchy. It’s risky. And it can b,eyed on spots that don’t absorb as well as others.
Sometimes the original tanning process was not done as well or they used cheaper chemicals and that will also create uneven absorption of dye or oil. Especially lower price range mass produced brands.
You might try taking the saddle apart by removing the fenders, take the stirrups off and really scrubbing the saddle down with mild soap (blue Dawn) and water rinse really well. That will remove dirt and other substances that will block absorbtion including old glycerine product build up over dirt. Air dry away from heat source. Apply multiple light costs of dark saddle oil allowing to completely dry between coats over several days.
As you apply each coat, watch for areas that are not going to take the oil or are sopping up too much. Adjust the amounts you are applying to those or, worst case, stop before you end up with a Pinto saddle.
Even a really careful saddle dyer or oiler can’t do a thing about varying absorption rates on used saddles. You can sort of ruin them even if they are still useable. I sure wouldn’t risk it and spend the money to possibly make a currently less popular color used saddle I wanted to sell even less desireable. Drop the price, let the buyer worry about it.