There has been plenty of sith who looked perfectly normal when gone to the darkside until they get disfigured someway
I think you just answered your own question with an insightful commentary on what tends to happen to those who trust in the dark side.
As for the game, as others have pointed out, the corruption is a customization option, much like the character creation sliders. You earn a certain amount of disfigurement on par with the degree to which you've been playing in the dark sandbox, and if you decide that your character hasn't actually had a disfiguring moment yet, you switch it off.
And at the moment that you decide they've now had such a thing happen...the full measure of the consequences of your power gone dark are waiting just under that serene mask.