This has happened to me a few times in the last few months on PS5. It usually happens after traveling or loading into a household and one of the sims is suddenly all black. Once it was a toddler who turned black, other times it's teens or adults, and none of them have tattoos. When this happens I just exit without saving since I don't know of any fix.
It just happened again now to a young adult sim and I tried having the sim change outfits, and his original skin tone came back. However, a few seconds later his brother in the same household turned black. It seems like when this happens the game doesn't have the capacity to load all of the sims' skin tones properly?