Genetics question -- SOLUTION
OK, now I feel really stupid and am going to go hide for several months.
On the advice of several smart and helpful people, I loaded a game that had exactly one piece of cc: the SimBlender. Before summoning Skye (the girl with the blue mouth), I popped into her family's home to see if maybe everything was fixed now. I sent her to the mirror to check herself out so I could see her mouth, and it was still blue. I decided "As long as I'm here, let me try changing her appearance one more time and see if that fixes it." I removed all makeup, and Boom!
A normal mouth. Then I had another thought: "Hmmm... Her eyeshadow was blue and sparkly... Her mouth was blue and might have been sparkly, who knows?" so I put the eyeshadow back on, and Boom!
Blue mouth. (Not the best picture, but you can see it a bit.)
So I quit without saving, replaced my entire downloads folder, loaded the house back up, sent her to the mirror to remove the eyeshadow in a game with cc, and Boom!
Normal mouth.
I use that eyeshadow rather a lot, and that's never happened before, but I still feel incredibly stupid and will now hide.
Huge thank you to everyone who offered help and advice!