I just now checked the black cars in the used car lots on 2 cycles of 700 days (day 3494-3500 and day 4194-4200, as that's where my 100% save is a right now, obviously I cheated to skip the days), and indeed they have the same mileage and price each time, so it's very likely that PD gave each used car a mileage. Probably at random, because that sounds like a massive pain. And a the 701th day, the list repeats from zero, just like it's been since GT1.
(PAL GT4 in French)
In GT2, used cars lots only read 3 sets of data: car model, price, color. GT4 most likely added mileage to that list from what I determined above.
In any case,
Black race cars are not unique models. They're only secondary paint jobs, just like any other special colored car, so if you were to buy them new, they'd be at 0mi/km. You can check that they indeed are not unique models, but secondary color by doing the color trick in Arcade mode. So if you were to find a way to buy any of the cars in their secondary colors, they'd be at 0mi/km.
Also contrary to many other people on this website, pez2k is a professional programmer that's been bringing leaps and bounds in terms of modding for Gran Turismo 2, and data mining, so he has a lot more credibility than you think he has. And in a lesser amount, so do I. So be a little more respectful of whom you're talking to please, because you never know who you're talking to.
Edit: I recorded proof of the fact that black cars are only a hidden paint on each car. People already have posted youtube videos of these, but I had to record mine to make sure.
So here's the Arcade color trick.