1993 Toyota Camry Black tranny fluid

I brought my car to a mechanic because it sometimes stalls at stoplights/signs. (Never did resolve that issue either). While he had it idling waiting for it to die, he was checking the fluids and found the tranny fluid was black. He says it means the tranny is bad or will go soon but I've never had a problem with it. Is this common? Is it worth it to flush and keep driving? Any thoughts/comments would be helpful on either subject.