There are a few reasons why I decided Ellen should wind up blonde in this comic, one of which being that Ellen's hairstyle doesn't really translate well to pure black. At least, not the way I ususally draw black hair these days. The same could be said of Justin's hair, Tedd's hair, or pretty much any hairstyle that parts and has a bit of lift to it.
Because of that, I actually decided on Ellen's hair turning blonde while burned out prior to coming up with a reason for why it would. I thought about it for a bit, and the last three panels of this comic are at least part of the result.
Of course, it didn't HAVE to be her hair that changed. I could have just changed her eye color! Granted, nobody would be able to tell, but it was an option.