Guess it depends on your definition of "original black plate car". As noted, the car would not have had black plates when it was new. But the plates were not replaced by California after 1969 or so (unless those are fakes).
Note that it doesn't necessarily mean the car never left CA, only that someone held onto the plates. I got a pair of black plates with my TR3, and was able to get them re-assigned to the car based on an old registration slip from 1975. Since I didn't get a title, there was no proof that the car was not registered in some other state in the meantime. And of course I now have a title.
PS Pricing seems maybe just a bit high to me, but not out of the ballpark. The "longdoor" adds a certain cachet, but there were some 4000 of them made so it's not a super-rare car. I paid less than half that for my TR3 with the original engine, original disc brakes (a big plus to me since I drive it regularly) and no big holes cut in the sheet metal. Only rust repair needed was under the stone guards.