Yes, that is true, they turn quite dark over time, I have a 4522-7000 that has had this happen. However to my eye this is all black and also (again to my eye) has a different finish to the dial surface, like there is a thick layer of paint / lacquer, whereas the original is very flat. Furthermore, the hour markers should be silver, not white, and the same applies to the minute hand.
Black ones are indeed quite common among eBay sellers etc, however they are not original. Many original examples have suffered water / moisture damage at some point, and have been repainted - sometimes black, as that is quite popular with buyers. Similar thing to what happens to vintage Omegas etc.
Here’s one that is all original:
https://thegrandseikoguy.com/product/grand-seiko-5645-7010-blue-dial/
Edit:
Accidentally linked a 5645 instead of a 5646, here’s the one I meant to refer to, however of course the dials are pretty much the same:
https://thegrandseikoguy.com/product/grand-seiko-5646-7010-2/Click to expand...