I am having the same problem, just picked up and am playing with the Growing Together pack.
A sim that I've been playing since a toddler (family made before the new expansion) aged up to a teen. She was fine until I opened the CAS to adjust her clothing. As soon as I exited the CAS, I got an error that said I was missing content and that one item in the buy mode was removed (though I've been playing this house for hours before this). This sim turned pitch black as well, and seems to be reset to a random sim. When I go into the CAS, she is fine (aside from a skin tone change - this was a base game skin tone, not a CC tone).
Every time I go into CAS, she is fine, every time I save her, she turns out black and her clothes/hair are randomized. If I revert her to a child state (using the full CAS cheat), and save, she is fine. When I age her to teen or older, she reverts to the black randomized sim.
UPDATE: I think I found the cause of it in my situation. The Sim child had a missing tooth when she aged up. This seemed to cause the black sim issue. Just on a fluke, I reverted her to a child state, and chose to "Fix tooth". Then I aged her up again, using the birthday cake. I then took her into the CAS as before and saved. When I came out of the CAS, she was fine!
Something about editing a teen Sim who aged up with a missing tooth caused the glitch for me.
