Well, I would fire that detailer, that's a certainty.
First thing to consider, how dirty ARE the seats? If they are only lightly soiled you can buy over the counter foam upholstery cleaners that you brush in then use a shop vac to slurp the cleaner up.
If they are really scroungy, it's a whole nother ball game and a real work out. You will need, a hose, a good shop vac, the tools to remove the seats, some PRO C-49, a scrub brush, and a whole Saturday. Take em out, vacuum them, mist them with the hose, spray them with C-49 diluted 5 parts water to 1 part cvleaner, scrub, vacuum the soap off as much as you can, mist them again, vacuum out the moisture and repeat until you get all the crud and soap off of them.
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Or... maybe rent an upholstery cleaning gadget, but it will do basically the same thing as above and cost you the rental price.
I'd be really curious about why the detailer said the seats would turn black. Could be that there is some ground in crud that he was concerned about wicking back into the fabric?
Kate