I have a ceiling in my bathroom that where the drywall is darkened (blackish) and flaking.
Will I have to tear it all down and start over, or is there any hope of a fix?
Depending on how bad it is, often you won't know untill you start removing the bad area. The real trick is to figure out what the cause is and repair that, poor ventulation, water leak above or what have you.
The problem was caused by poor ventalation.
Post a photo we can tell you if it's minor or not.
Do you have any windows? Fan?
If not maybe installing a exhaust fan might be in best interest.